I have tried different wifi networks, with different broadband providers, with no improvement.
I do not have other SIM cards to try in my handset, but other's are able to use their identical handsets (with SIM cards from other providers) on the same wifi networks without problems, i.e. they can send and receive SMS messages using Wifi calling on the same wifi connections that do not work for me.
I have put the SIM into a Samsung S9+, iPhone 11 and an iPhone SE (2016) and i can make and receive calls and send text messages when on Wifi calling but cannot receive text messages. As others have reported it can take several hours for messages to be received once in an area with better network coverage (when Wifi Calling can be turned off).
This is a show-stopper as it is preventing me from porting my mobile to Virgin Mobile as the ability to receive text messages is vital in these days of on-line shopping and identity verification.