Reboot the client when the server is up again. Strange that mount -a doesn't work...
I use autofs. I don't involve OMV at all in mounting remote NFS shares, only in providing the shares.
I have four OMV servers. They all provide NFS shares and they all mount each other's share using autofs. Works fine. Very stable. If a server is disconnected or rebooted, the mounts are dropped and restored on demand automagically.
You might prefer to use systemd.automount instead of autofs. It is the modern way. But I don't know if the OSMC player supports it. Or even if it supports autofs.
Thanks for the tips Adoby.
Yea I tried rebooting client several times and no joy. Only way I can remount is by deleting the NFS share in OMV gui and then adding it in again.
I'll check out autofs. OSMC is based on Debian so hopefully should be ok.
Should have mentioned I'm using systemd.automount with fstab currently on client.