If you want a stable static IP address regardless of whether you are using your ADSL/VDSL or a LTE fallback, look at the Andrews & Arnold L2TP service. £10 per month with a 1TB monthly limit, static IPv4 address and block of IPv6 addresses, configurable rDNS etc (all the things you'd expect from A&A). It costs a bit more for a VAT registered company, but still excellent value - it makes a Plusnet connection almost like a business level service.

I use a Vigor 2860n+ with the L2TP tunnel set to prefer my Three LTE connection (using a MikroTik SXT LTE connected to the WAN2 of the Draytek)), but it will swap to the ADSL if Three is disconnected for any reason. You can do that the other way around if you prefer.

https://www.aa.net.uk/broadband/l2tp-service/

Guides; https://www.expressvpn.com/support/vpn-setup/draytek-drayos-l2tp/