Connections dropping due to packet fragmentation

I’ve spent a good while trying to troubleshoot a machine that I can connect to via ZeroTier fine.
It can be pinged, ssh connects, you can login, type commands etc.
As soon as you use a command that generates a lot of feedback, the connection will drop.
Same for any protocol e.g. loading a web page of a few hundred bytes works fine, anything larger, the connection drops.
Ping would stop working if the payload size was over ~1350 bytes.
Tcpdump on both ends would show packets getting fragmented along the way.

Is ZeroTier incorrectly detecting the mtu of the internet connection?

I was able to make it work by adding these lines to local.conf:
“physical”: {
“”: {
“mtu”: 1400

Is there a better solution to this?
If not, hopefully this will help anyone else having the same issue.

Thanks for writing. Good tip.
What kind of ISP is this happening on?

A 4G modem.
It’s been working previously and then one day stopped.
I was trying to find out where along the route the packets were getting fragmented but 1500 byte icmp packets make it through without being fragmented so it’s difficult to work out.

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