@terrbear, I followed your lead (set MTU in ORBI to 930 bytes) and now the Disneyplus and TVNZ+ apps on the TV work. TVNZ+ is not 100% but is mostly OK.
Also, www.westpac.co.nz & bank.westpac.co.nz where the hitting the login button takes you, are now working from the laptop.
It is not a fix as AnyDesk sessions to the laptop are a bit glitchy and there are other signs. However, it does provide more information for the 2degrees engineer to work with.
For everyone else, We are not in the Whanganui area!