Can you call out but not receive incoming calls ?
If so not sure if that is a wifi calling issue or the mobile internet. My phone is too old (Android10) but my wife's Samsung (Android 14) is set up for wifi calling. However we have just changed to 3 using same unit as you from Plusnet FTTC which used their Hub 2. Wife could always make calls using only wifi calling in Airplane mode but often incoming calls would go to Voicemail. To get round this she keeps Airplane off with phone set to use wifi for data when available before Talkmobile data as she has a 30Gb monthly limit on her contract.
It might be that a mobile router makes contact with the nearest tower less often than a landline or fibre-connected router does when in standby mode which might then cause incoming calls to go to Voicemail. Someone with inside tech knowledge of these things might understand better than me.
Having a phone in Battery Saving Mode can also upset the reliability of incoming calls sometimes and maybe even worse if using wifi calling too.
