I just thought I would come back to this to update with what I think was the issue and the possible solution. Because the use of the old MBP was critical for my son's homework, we ended up connecting...

@David-TP Thanks David. Yes, I saw that and tried those steps. Interestingly when on the Guest network it connected to 5Ghz (it was both enabled). I did wonder if it was a frequency issue so tried...

A really strange one, this; recently had to replace a defunct Tenda setup with the Deco PX50 setup - three nodes which are used solely as wifi access points connected to an existing ISP's router with...