So, in summary looking at the
mount -v output we see that this nfs server is not able to talk nfs4 but nfs3 fails since the rpc communication is borked. So, the error seems to be outside the OSMC device.
I would check the output of
rpcinfo -p 127.0.0.1 on the Fedora system to see what has been registered and is available. Also check the netmask of the used IPv4 configuration on both sides but now I’m running out of ideas for the moment.
Others are welcome to help nickc with his router and Fedora NFS server to get it run.