Is it possible to force an app to use cellular data instead of WiFi?
As far as I know, android won't use cellular data at all when wifi is enabled. On a regular machine I'd create routes for whatsapp to use the celular interface, and have a default route using the wifi interface.

One solutio might be to use a VPN server at home, and connect to that on your phone. If you run OpenVPN op tcp port 443, then you'll probably successfully evade the firewall. The downside: it can eat battery like crazy (at least, it did when I tried).