Even in bridge mode you can still be slowed down by the modem if its CPU can’t handle your traffic. That’s unless the isp modem offer a complete passthrough. That’s what was happening to me even in bridge mode where I was getting my own IP through pppoe. The modem couldn’t be made into full passthrough and was hitting 100% CPU.
First thing I’d do is to look at the client (fedora) journal for anything funky happening.
‘sudo systemctl status nfs-client’
Since it’s random I assume you won’t have any timeout in your /etc/fstab but it might be worth taking a look anyway.
Be aware that if the network drops the NFS will be disconnected and won’t auto-reconnect so this could also be the issue.
I don’t know if it plays well with container mounted volume, but looking at autofs could be a solution to auto-remount the share. I use it profusely for network mounted home directories.