iPhone to Facebook Portal video problem

For over 12 months, I've been able to video call using WhatsApp between iPhone and Facebook Portal ..but now now. Now, when making / receiving a video call, the video on the screens on both devices freeze, or just update about 1 frame every 5 seconds. On the iPhone, the 'quality of connection' dot flashes between orange and green constantly.

I don't believe it's a wifi issue, as I've tried both the Portal and iPhone on other wifi networks. I've also tried the iPhone on 4G. I've tried other iPhones, and it's always the same.

The iPhone(s) are all on iOS 14.6 and the Portal is up-to-date with software too ..in fact I originally took the Portal back to the retailer and they gave me a new one ..and even after downloading the Portal updates the problem remains.

I suspect it's an iOS / Portal software clash - but who knows! I've checked 'low data mode' and 'use local network' settings on the iPhone(s) ..none of these guilty.

Does anyone have any experience of this problem please, or any suggestions?

Many thanks