Free probably piggybacks off one of those you list - you need to ask them for the settings not here!
And it seems you have already found where to put the settings in the ARM300m
good to find another device using default 192.168.8.1.
Yes this is a problem. We have this problem as well because we have a lot of our own device and we have to change LAN ip manually.
A strategy is to change own IP address if wan IP is in the same subnet. But need to visit the page using domain, not using IP. Also need to consider about bridging.