Problems accessing remote host over cellular

[mrmortar]

Hi there,
I've recently setup WireGuard on a PiZero so I can use it for my VPN when out and about using public Wi-Fi. It also works over cellular which has been useful.
The challenge i'm trying to overcome is why I can't connect via VNC to my home PC through the Wireguard VPN once on cellular. I can ping my PC but nothing more. If I connect to Wireguard locally (whilst on my home router) I can connect to the PC.

So in summary:
Device client > at home Wireguard VPN enabled > remote PC with VNC server = works.
Device client > on celluar with Wireguard VPN enabled > remote PC with VNC server = connection failed (but can ping it).

**update - I've tried connect with an FTP client to my PC that has FTP hosted and I can see in the logs is doing something but ends up closing the connection.

Smells like a firewall issue but not sure.

Comments appreciated.