AsusWRT Merlin

WRT style modems have always been great for bolting in third party firmware but i've really struggled to find a really decent model for cellular bonding (you have to go marine/ship!) but the closest to having a decent custom firmware router might be to use something like this -- i've always liked ASUS kit.

a third party alternative firmware for Asus routers, with a special emphasis on tweaks and fixes rather than radical changes or collecting as many features as possible