I don't think there's a hard and fast rule for MRU settings. Seems like it can be different and getting the right setting is a bit of trial and error. I did this which worked for me
I know this post is a year old but I'm having the same issue. I have the BT smart hub 2 with a fibre 2 contract so all I have the the actual Smart Hub 2 which is connected to phone socket.
However, I have my own TP Link Archer AX5400 Router and I want to use this as I'm pretty sure it's a better router than what the smart hub 2 provides. How do I set this up so that my wifi comes through the TP Link router instead of the smart hub?
Any help would be appreciated