I've had the same Plusnet mobile SIM for a few years now but have never been able to send or receive MMS messages. I have tried the standard APN name fix mentioned a few times on these forums to no effect and had wrongly assumed it was the fault my phone (Xiaomi Poco F1). However, I have just upgraded to a Nothing Phone (1) and have the same problem. I've tried the same APN name fix, but again it hasn't done anything.
I'm on a 10GB, unlimited texts & minutes 30 day contract, with plenty of data to spare.
I've attached screenshots of the APNs, of which the ones labelled "Internet" and "MMS" are the defaults, and "EE Internet" is the one I created as the supposed fix.