HUKD persuaded me to go with the 3 MBB, and this place persuaded me to get the B525 - so here's my go at giving something back... This doesn't depend on you using 3 - but I guess it's the current favourite given the price point.
Finally got around to "packing" something together to enable bridge mode + a lot more extra firmware options (such as selectively choosing what bands to use - I've actively disabled band 20 due to the speed decrease if it swaps to it).
My current setup uses this, teamed with a Virgin 350Mb connection. There's a slight overall speed increase, but it's the failover aspect which I was interested in and it works brilliantly.
This is only for the B525-23a. This model can be had cheaper and gives you all the bands in use in Europe.
Honestly, now that 5G is in full swing there is no benefit to the -65a in the UK/Europe as no new 4G bands are going to be certified. This firmware gives an awful lot more to the interface too, not just Bridge mode.
The big thing if you're doing this is you will need a USB-A to USB-A cable. This is a completely non-standard cable and doesn't conform to USB specifications. Fortunately, these cables are available from China or you can make one yourself. It's just how you access the firmware mode of the Huawei kit.
Firmware pack is here: mega.nz
All the info is in the readme file. Make sure you extract the full archive before running the commands.