Fixed it myself somehow.. I think "restore" option (not factory restore - just what it got delivered as) may have done it as I could select AP Mode on the initial setup wizard.
Interestingly the EB810v SFP+ port did actually work, but would only negotiate at 1g with Ubiquiti DAC cable (UDM-Pro SFP+ DAC -> EB810v SFP+ DAC) - the EB810v port was actually working as a WAN port. Perhaps with a different module it could negotiate properly as there is no setting in the TP Link UI for it
I changed to using the SFP+ RJ45 module on the UDM-Pro and it was fine at 10g to the EB810v WAN RJ45. So now have it in AP Mode with an UDM-Pro in the middle, so it looks like this:
Code:
ONT 10GBASE-T -> UDM-Pro SFP+ 10GBASE-T WAN
SFP+ 10GBASE-T LAN -> EB810v 10GBASE-T WAN in AP Mode (using its 10GBASE-T LAN port for local server)