L2 bridge, traffic flowing in just one direction

Hello, I have a fairly simple setup:
One LAN with 1 host, 1 gateway (Teltonika) with zerotier configured with L2 bridging (activated in the zerotier console), 2 external machines with static IP on the L2 address range,

The two external machines can ping each other and can be pinged from the Teltonika and from the LAN host, but cannot ping neither the Teltonika nor the LAN host.

On the Teltonika, zerotier-cli peers shows all the machines.

Any suggestion where to look for ?

Thank you

One note: this happens when connected via mobile network (but sometimes it works); when the router is using wifi to connect to the internet the problem disappears.

It seems an MTU issue, but there is something going bad with Zerotier on mobile.
Default MTU of 2800 prevents traffic on mobile networks from Teltonika router with different providers.
We checked the mobile connection MTU pinging google. and the mobile network MTU is 1416, average latency is 40 ms and packet loss is 0.
Lowering the Zerotier MTU to 1416, allows traffic in both directions, but packet loss is high (50%) and latency is high and variable (from 100 to 600 ms).
We tried to lower the MTU to 1280 (the minimum allowed) and things get a bit better.
There is something going bad and causing packet loss and retransmission.