Cool yeah that would work. Obviously requires some more pieces of kit but worth thinking about - thanks for the write up!

I know it is possible if you use something like ROOter (Openwrt based) but MR600 isn't on their supported list unfortunately https://www.ofmodemsandmen.com/routers.html It's a pity since they've got...

@Fowlet did you ever get this right? I'm in a similar position - when it works the 4g connection is preferable but I'd like to switch over to the more reliable DSL connection (available over LAN)