When connected to carplay the phone goes offline and will not use cellular data even though wifi assist is selected

I have an aftermarket android head unit using zlink to connect to carplay.

It connects via bluetooth and then through wifi.

For the first year that I had the unit Waze was live and I could search new addresses etc. Or I could google something. Or use spotify to play something I hadn't downloaded.

For the last 2 years the Iphone 13 will connect but Waze etc will be offline (95% of the time) and I cannot use any mobile/cellular data. The carplay unit will show a wifi signal strength icon. The phone itself cannot connect to the internet until I disconnect from the wifi network for the carplay. I can then use the phone as usual.

Sometimes the 5G connection stays on. When this is the case a '5G' icon replaces the wifi icon on the head unit.

A couple of times I've noticed a message something like 'wifi connection has no connectivity, use mobile data' (to paraphrase) and when 'ok' selected the phone will be live again. But this is only when I've pulled over to google something or someone else has been trying to use my phone. Under a handful of times.

Wifi assist is on.

Any insight into what to do to maintain the mobile/cellular data connection even though the phone is connected via wifi would be most appreciated. Even if there is some sequence I could follow - I already make sure I select my waze destination before turning on the car so that I can get live traffic info at least initially.

Thank you