i ran into something slightly similar with usg (before moving to pfsense) -
the ISP had given us one of those combo units, and the TV worked through MoCA (not sure if this is what you mean by IPTV)
was entirely not compatible with USG (at that time).
got a dedicated modem, no wifi, usg was still gateway after that for a while (until i started wanting more funtionality that unifi just does not offer)
to my point, our cable tv would not work after switching to modem only. we ended up having to replace the cable boxes as they entirely relied on MoCA (edit additional - new modem did not have MoCA)
im guessing you want to add vlan tags to pfsense interfaces to get it to pass-thru in a sense?
my knowledge on VLAN specifically isn't that high up, but from what I understand, you'd likely want that vlan tagged to be allowed on wan, and also on the port supplying connecting facing your unifi?
i feel like i missing something, but hope this helps