You may find that your ISP won't support other routers, especially when you're using blocks of globally routed IPs.
I've run into multiple providers here in the US that /require/ you to use their specific router for blocks of globally routed IPs (which are only available on business class subscriptions).
People have talked about replacing the routers with something else and maybe getting it to work. But this is an unsupported configuration and they accepted the risk of doing so.
I've heard / read accounts of cable internet providers in the US doing this and requiring their router, which did standard things, but they didn't tell people. Supposedly their router used RIP on the CMTS side to advertise the local globally routed block to the upstream CMTS routers (which didn't advertise anything). Thus the upstream routers could find the downstream router that had the block of globally routed IPs, despite the fact that said router was itself using DHCP from the provider.
In some ways this seems like a nasty hack. But in other ways it seems like an elegant solution.
You may end up being tied to the "super" router provided by the ISP. Or an unsupported solution.
Good luck.