4G/LTE failover setup/doubts

[jaclaz]

Lately  I had a couple of (temporary) failures of internet connection(s) at the office, the first one lasted a couple days, the latter only a few hours.

Nowadays we have a lot of programs/devices that (stupidly and in some cases without any real need) cannot operate without an internet connection, besides - of course - the lack of e-mails that is an issue.

We had the same thing happen a couple years ago, the downtime was much longer, possibly 4, 5 or 6 days, as while doing some roadwork nearby they cut both the telephone  and xDSL  lines in several points (the demented guys were inserting posts for guardrails and managed to physically cut the cables every 3-4 m for some 200 m before the telephone company managed to find out and stopped them, so they had to re-dig and replace some 250 m of cables).

At the time, in emergency, I put together a temporary solution (half-@§§ed and el-cheapo as always).

For telephone we used a cellular phone, managing the telephone company to temporary divert the landline number to it.

For internet I got one of those Huawei portable wi-fi "modems"[1] (I often happen to ask myself what - the heck - is actually modulated o demodulated in the devices that the industry insists on calling "modems") with its SIM,   and found a device from Link-sys that can act as access point and wi-fi bridge, and a couple other functions.

So, connected it to the LAN (if I recall correctly it seemingly becomes "transparent" in it when in "bridge" mode) and directed it to bridge to 192.168.1.1 via wi-fi, then set the soap bar own IP to 192.168.1.1, disconnected from mains/switched off the "normal" dsl router/modem (as well normally set to 192.168.1.1) and - more by sheer luck than because I knew what I was doing - everything worked (though slowly, on UMTS connection, anyway enough for the *whatever*  needed internet, including e-mail) and ALL devices on LAN needed not to have their gateway - fixed to 192.168.1.1 - changed.

Now, I would like to set up a ( still el-cheapo, but possibly not-so-half-@§§ed) more permanent/more suitable and possibly automated solution for these emergency cases.

In my perverted mind, this (failover to LTE in case DSL connection is down) should be one of the simplest/most straightforward things to do, BUT I cannot understand, or trust, or both how exactly to do it.

My LAN is as simple as it can be (there is NO, nor I want any wi-fi[2]):
Various devices and PC's [IP 192.168.1.xx/255.255.255.0]
|
Unmanaged switch(es)
|
Current DSL modem/router [IP 192.168.1.1/255.255.255.0]

Now I have found three possibilities :

1) replace the DSL modem/router with one that has a provision for failover to a USB "internet key" (that contains the SIM)[3], i.e.:

Various devices and PC's [IP 192.168.1.xx/255.255.255.0]
|
Unmanaged switch(es)
|
NEW DSL modem/router [IP 192.168.1.1/255.255.255.0] -> failover to USB "internet key"

2) insert between the switch and the current DSL modem/router a dual WAN router with failover functions and add a LTE/4G modem or modem/router. i.e.:

Various devices and PC's [IP 192.168.1.xx/255.255.255.0]
|
Unmanaged switch(es)
|
dual Wan router [IP  192.168.1.1/255.255.255.0] -> WAN 2 set as failover -> LTE/4G modem/router [IP DMZ 192.168.2.1 - gateway 192.168.2.254/255.255.255.0]
|
WAN1 set as main connection -> Current DSL modem/router [IP DMZ 192.168.3.1 - gateway 192.168.3.254/255.255.255.0]

The dual wan router I could use is the Ubiquiti Edge Router X, that seems like cheap and reliable.

3) insert between the switch and the current DSL modem/router a LTE/4G modem or modem/router i.e.:

Various devices and PC's [IP 192.168.1.xx/255.255.255.0]
|
Unmanaged switch(es)
|
LTE/4G modem/router -> failover to  [IP 192.168.1.1/255.255.255.0]
|
WAN 1 main connection -> Current DSL modem/router [IP settings???]

For the LTE/4G modem/router I have EXCLUDED the Netgear LB2120 (as it seems that - while being extremely popular - half the internet is reporting that is working just fine, exception made for its failover functions) so candidates are the (preferred) Teltonika Router RUT240 (that seems to have a very good reputation) or *something* else, like one of the n LTE modem routers with wi-fi (that I have no use for) and automatic failover, an example is the digicom R4GW-T04.

Now the questions:

1) do you have any experience with similar setups?
2) if yes, suggestions about suitable hardware (4G/LTE modem/routers with or without failover, devices that double as dsl and LTE/4G routers, DSL routers which allow use of a second WAN for the LTE/AG modem/routers and similar) are welcome, what do you use?
3) in the specific case #2 I have found a couple (IMHO vague enough) examples where the modem/routers have the 254 address but are set in the dual wan routers as DMZ with address 1, but I am not sure to understand the logic/method  used, if you have some good resources/examples/tutorials, they would be appreciated, as well as any correction, possibly what I found is specific to the Ubiquiti router only and other devices work differently. Or, have you better ideas for the dual wan router and its settings? 
4) in the specific case #3 above listed, how (the heck) are to be setup the IP settings (if any) on the "old" DSL modem?
5) any other ideas/examples/explanation to get to the same final result (adding a reliable failover to 4G/LTE to a simple LAN connected primarily via a DSL modem/router)?

Please note how the "el-cheapo" characteristic is vital, so please avoid Cisco and similar high end, professional, suggestions, we are talking of a 100-200 € budget here.

jaclaz

[1] we affectionately call them "saponette" in Italian, i.e. soap bars
[2] actually there is one, that *somehow* I manage to maintain working, that uses however an own set of access points connected through a VPN and an internal router, so - for all that matters - the whole stuff is only a single device connected to the switch 
[3] BUT these "internet keys" have - at least the ones I have used in the past - relatively low connection speeds and will be poorly receiving in the place where the DSL modem/router is placed, AND it is pure madness as there is no guarantee whatsoever that the USB "internet key" will be compatible with the modem/router, nor that they will be compatible with this (or that) low cost SIM provider, so possible solution #1 is excluded