Forgot to say thanks, sorry!
In the meantime I've chosen to upgrade to another tiny but a lot faster device and I didn't need to do this modification to make it work as expected.
I'm just having a lot of
read UDPv4 [ECONNREFUSED]: Connection refused (code=146)
and
read UDPv4 [EHOSTUNREACH|EHOSTUNREACH|EHOSTUNREACH]: No route to host (code=148)
in the logs for an unknown reason but I'll open another thread for this!