HA not working behind firewall (only with iPhone) - no reverse proxy

Configuration
1

I’m trying to configure HA to be accessible from the Internet using a simple NAT on my firewall (OPNsense).
This is what happens:
if I connect to http://ha.mydomain.it:2087 from my iPhone in 4G, works perfectly,
if I connect from my pc on WiFi to the same url, it doesn’t load the page. I get a blank page and it keeps loading for a minute or two, but after that nothing (not even a timeout).

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Please help me, I’m out of ideas.

INFO:
Home Assistant has been installed as a virtual machine on my Proxmox VE.
Home Assistant version 2022.2.5