Hi Weaver,
This is my first posting on this forum!
I do use 4G myself, and *load balance* (not bond) it with my slow-ish 2.5Mbps ADSL connection. I'm not sure if true bonding is usually available for Fixed Line + 4G connections, as that has to be done at both the user's end plus the ISP end, as far as I know. So load balancing (+ "fail-over") seems the next best way of combining the 2 types of connections - the slower fixed line connection provides stability and unlimited data, and the 4G is useful for "turbo charging" for bursts of speed when you need it (plus v.useful backup connection).
I live in rural Ceredigion (in West Wales), and unfortunately we've been left out of any BDUK upgrades (via "SuperFast Cymru" here) - and being on a long-ish EO (Exchange Only) line, options are few & far between.
Also, being in a small hamlet (<= approx 30 premises), and sandwiched between villages that have been upgraded to FTTC, this has resulted in splitting the area into smaller fragments of slower broadband (usually <= 3Mbps, all on EO lines), making community schemes less viable.
I'm still going to look into possible options: Openreach Community Fibre, FWA [Fixed Wireless Access via a local WISP], and TV Whitespace are the main possibilities I can think of, but with <= 30 households in play, and most providers needing at least 10 or 20 properties to sign up to make it viable, then as I mentioned, community schemes become a bit harder.
Hence, the option of a 4G connection. I use a DrayTek Vigor 2860Vac router (with built-in ADSL/VDSL modem) as my main router, and a TP-Link Archer MR200 4G router with an EE Data SIM (32Gb currently, but now looking at the >= 100Gb per month data offers from Virgin / EE and testing Three coverage too), and the DrayTek has good options for load balancing these 2 connections, with plenty of scope for configuration which links to use for what (I haven't really got into it fully get).
So, if as I understand things Weaver, you have several ADSL lines bonded together, you could add a 4G link load-balanced with that connection. You may even find that you could drop 1 (or 2) of your landlines eventually if all goes well, saving you money.
I'm thinking of upgrading to the Huawei B525 4G router soon, as that's a Category 6 LTE router and, testing with a friend in the village who has one, gives a better connection than the TP-Link MR200. Also, I have a Poynting XPOL-A0001 omni-directional external 4G antenna that I may use, although I haven't really needed it as yet.
But, in summary: I think a 4G connection is definitely worth adding into the mix if your only other current option is ADSL - well worth experimenting with at least. There are some good deals with EE or EE-based mobile networks at the moment (e.g. Virgin are doing 120Gb per month for £20 per month until 31 January - see:
https://www.ispreview.co.uk/index.php/2019/01/virgin-mobile-uk-offers-120gb-data-4g-sim-plan-for-just-20.html - and Three do unlimited data for around £27 per month - but you have to look out for special offers every so often, e.g. "Black Friday" deals in November, and at other times too (I've seen Three with unlimited data [well, up to 1Tb - see other post in thread referencing ThinkBroadBand forum thread] for £20 per month, and Virgin Mobile [via EE's network] with 200Gb [or even 300Gb] for £20 per month). It's a good idea to get a decent 4G router, and order a few test Data SIMS (free from the mobile provider websites - Three give you 200Mb free data per month, which is plenty to test coverage and run speed tests, with other networks you may have to top-up with some credit to run speed tests, but even then you can still get a rough indication of signal strength from the router LED lights - remember to force router into 4G mode so that it doesn't fall back to 2G or 3G), so that you can test network coverage and speed in your area.
4G (at least via EE [=BT]) will have to be a suitable alternative to FTTC (with enough backhaul), as that's what BT/Openreach are proposing to use to cover the majority of USO (Universal Service Obligation) premises (see BT's response to Ofcom's USO consultation where they mention using their 4G FWA to cover the majority of premises, and also mention they will monitor & upgrade 4G capacity as & when needed - link to PDF file:
https://www.ofcom.org.uk/__data/assets/pdf_file/0013/120361/BT.pdf).
Good Luck anyway! It's definitely worth at least experimenting with 4G, I would say.
EDIT: I forgot to mention, I get consistently over 10Mbps downstream & 1Mbps upstream via my 4G link, which is the minimum I'm looking for (based on proposed USO) - I usually get 15-30Mbps downstream and 3-15Mbps upstream (I'm expecting the Huawei B525 router will improve on this, testing over next 2-3 days). I have had downstream speeds up to 50Mbps on a few occasions! And using an external 4G antenna could improve speeds & consistency even further. I've just used the 4G router built-in antennae up until now (which have much better signal reception than any smartphone I've tried).
All the best...
Adam.