Not sure on MergerFS…I haven’t gotten deep into ZFS pools.

What ZFS are you using?

“As of November 2023, this feature was merged in main and is scheduled for release in OpenZFS 2.3:”

'The OpenZFS project (ZFS on Linux, ZFS on FreeBSD) is working on a feature to allow the addition of new physical devices to existing RAID-Z vdevs. This will allow, for instance, the expansion of a 6-drive RAID-Z2 vdev into a 7-drive RAID-Z2 vdev. This will happen while the filesystem is online, and will be repeatable once the expansion is complete (e.g., 7-drive vdev → 8-drive vdev)."

Awesome! Interesting with MTU. This was a first hit google search, seems MTU in wireguard setup plays a role also. https://gist.github.com/nitred/f16850ca48c48c79bf422e90ee5b9d95

It may not even be the issue, I’m just spitballing, and it maybe DNS issues like others mentioned or cell provider blocking some aspect. As you probably know Wireguard can be set for all traffic to route through the tunnel, or some outside of it. Maybe cell network is not routing DNS through the tunnel, but using its own, or maybe they used a cached Domain name lookup that doesn’t have your domain IP. Again, networking/DNS etc is my blind spot. I.e. I have setup openmedia vault on a 256MB RAM arm board to serve my music and SMB shares, but I don’t understand reverse proxy LOL.

I don’t have a lot of networking skill here, but could it be your WiFi connections (anywhere) are IPv4 and on mobile it forces IPv6, thus why you can connect via IP:port? Typically your wireguard host machine has IP forwarding/masquarading setup so you can reach your home LAN. Could this be an issue through the router / proxy?

Yes, but No, but. It’s like an always on self discovering VPN. No need to connect and login if you lose connection or change from WiFi to cell to Ethernet, it just figures it out. And as other commentor said it is wireguard. So you can set it up yourself without a 3rd party, just takes a little bit of tech savvy skill and trasfering some public keys between each set of connections. Tailscale just makes it effortless.

If it is a one time transfer, plug them both into a PC ,set USB as file transfer then copy the folders over

Does this count ARMv6 256MB RAM running OpenMediaVault…hmm I have to fix my clock. LOL

Something like fslint might be what you want. Scan folders, lists duplicates, you set how you want to deal with them. Its more manual, but I think it is what you are actually trying to achieve.