de0u I tried accessing from vanadium once and it connected. However, the second time (right at this moment) I tried and it gave me a quick answer of ERR_ADDRESS_UNREACHABLE (indicating in my interpretention no presence of firewalls whatsoever).
As I was typing this answer, I refreshed the servers page on vanadium and it happened to have again access to the server.
As far as I can see, there is some rule in place, somewhere I can't tell, that is not letting me reliably access the local network from my phone.
Worth mentioning that this is only happening on my phone: when the phone looses connectivity to my local network, all other devices don't.