tl;dr FTTP. A single outage event in 16 months, lasted 40 minutes, whilst asleep.
I'm in the UK and have FTTP through BT. Way back when I also purchased the 4G backup option (Hybrid Connect) that comes with this service. That's an extra £7.50/mo when taken as part of the usual 24-month contract. It's simple to setup and doesn't require any specific maintenance.
Looking back at the logs it's clear (from an actual usage perspective) it's not been worth the £7.50/mo I've been paying for it, but I'll admit it helped give me peace of mind when I was on-call for work so it is easy to justify.
The BT supplied router (which is required if you want to use their 4G backup hardware) keeps a log of "Resilience events".
In the 16 months I've had FTTP it has had exactly one "resilience event". A 40m11s outage that started at 00:20:05 on 28/11/2025. I was unaware of this outage as I was asleep.
It was really useful when I moved house though. I was in the middle of a 24-month contract with BT at my old address so I ported my contract to my new address. This meant they had to come round and install FTTP at the new property, which they couldn't do for a couple of weeks, so I was without home Internet for these two weeks.
Luckily the 4G "Hybrid Connect" backup device wasn't geo-locked (or if it was maybe BT overlooked it given there was an outstanding "Moving house" order on the account) and so it worked perfectly for the ~2 weeks between moving in and FTTP being installed. If this hadn't worked then a temporary 4G router would have worked just as well.
I had a bunch of "resilience events" for this period (it wasn't one continuous event as I was moving/restarting the broadband router for various reasons). During those 13 days the logs show 163GB download and 25GB upload. That's an average of ~150KBps (note the capital B there, in bits-per-second it is ~1.1Mbps) download.
In the 26 months prior to moving (where I was on ~75Mbps FTTC with BT) I had 3 "resilience events". 17m36s, 47m7s, 31m22s. All between midnight and 4am where I wouldn't have noticed, these were also within 1 month of each other, the other 25 months had no problems at all. None.
When my current contract comes to an end I'll move to a different supplier (probably Community Fibre as I can get symmetric 5Gbps for less than I'm paying BT for 1Gbps/120Mbps) and then not worry about a backup. If it is less reliable then I'll look for a solution then.
My current backup is simply to hotspot on my mobile with 5G (good signal here). Doesn't help the others in the house but they can fend for themselves. Neighbours have different suppliers or technologies (DOCSIS vs FTTP) so swapping wifi details would also be an option.
As others have pointed out, a local power-cut that takes out of the FTTP cabinet could easily take out the local 4G/5G masts making a 4G backup solution useless. If this happens I can just take my laptop to a nearby cafe or the co-working space I use. That kind of outage is very rare though round here.
Then again with the sums involved (under £10/month extra) it may just be easier (for peace of mind again) to just plump for something that doesn't really make amazing financial sense as it's just the cost of a pint or two a month.