I've just tried to replace a BT Biz bub with an external 4G USB failover with a Vigor 2862ac -
2 things are causing me trouble.

The router reboots itself 3-4 times per day - tried disabling HW acceleration, 5GHz Wifi, IPv6, factory reset and reconfigured manually, replaced power supply but it still reboots.

i can't get the 4g USB modem to work taht BT supplied as a failover with the Biz Hub. It appears to connect in DHCP mode and picks up an IP address and i can ping the gateway address it gets too but no internet traffic through it - tried user & PW eesecure, secure, ee, APN everywhere, btmobile.com and every permutation i can manage but not traffic through it. Anyone else managed to get one to work??

I had one working with My 2762 out of the box. I have a 2862 now as I'd like to do load balancing (it works with that too). Have you looked at the admin page for the USB stick on 192.168.8.1? It's an RNDIS device so it basically pretends to be a usb ethernet adapter connected to a router. I think it should be in "3G/4G Modem DHCP mode" and you shouldn't need to fill in any details in the router like APN of username, that's all handled in the stick.