Surely the simplest approach is not to use a wifi ssid that you don’t trust. Yes, mobile broadband is slower and more expensive, but is also more under your control, it is however harder for your boss to monitor.

If I was an it manager I’d probably want to keep an eye on WiFi traffic, even on the open ssid, just to see what’s going on. I don’t trust free WiFi.

If you can’t use 4/5g, and can’t find a suitable firewall or VPN, can you set up a different user on your phone? - and be disciplined in is use.

Has a psion5, brilliant tech. Loved it. The calendar was way ahead of its time, 2AA batteries gave weeks of life and you could just about touch type on the keyboard. If I could get it to synch with office I’d be tempted to dig it out again, it really was the future.