No forwarding needed, the devices always call out. I’m a facilities guy too, not IT, but I went through this at our hotel last year when our IT contractor put all the room sensors and the boiler gateway on a separate SSID, and I learned a lot from that.
What the sensors need outbound:
TCP 8883 MQTT over TLS to the device host on your Connection details card
TCP 443 only if any of them use the HTTPS device API
UDP 53 DNS (they connect by host name, TLS needs the name)
UDP 123 NTP, the ESP needs real time to check the server certificateNothing inbound. On the AX55 guest network with “see each other and access my local network” switched OFF you get roughly the right thing: the ESPs can reach the internet but not the laptop.
My bet for your test sensor: either it can’t resolve the host (some guest networks hand out odd DNS), or it can’t get time. Ours did exactly that, the ESPs sat there failing TLS because NTP was blocked on the new SSID. Watch the serial output if you can.
On the Hermetry side, open your device profile and check the hardening list on the device transport security page: plain MQTT off (TLS only, port 8883), token in the URL off, commands allowed without TLS empty. There is also an Allowed networks field (up to 64 networks or addresses); devices outside it are refused. If your home has a fixed public IP you can put it there, so a token copied off a board is useless anywhere else.
For your meter page: with isolation on, your phone on the main Wi-Fi won’t reach it. We kept one laptop on the device SSID for that kind of thing.