I have just changed ISP from the amazing Andrews and Arnold, to Gigaclear, as they are the only fibre offering in my village.
There is not much online about not using their Linksys kit as supplied (after looking into it, I didn’t really like the look of them and they are limited to 300mps over wifi, and I’m paying for 500mbps. Nor do they seem to publish anything in the way of instructions for using third party routers, other than saying they offer no support.
In case it helps anyone, Gigaclear (weirdly) just hand out IP’s via DHCP so change your router settings to reflect this and you will be good to go, there is no login or protocol, just blindly set the router to pull an IP and it all just works.
I don’t think they handle IPv6 yet. I should move from their DNS but I am letting everything settle down before I start meddling. These are my Ubiquiti UDR UniFi Dream Router settings: