I’d suggest having one mqtt gateway node per physical mesh. The easiest/most stable way is to
1: factory reset an esp32 based node like a heltec,
2: get it setup on your wifi under the radio config: network menu. After it reboots, you can cycle through the screens to find your IP, or check your DHCP clients on the router to find the IP to connect to connect in the Meshtastic app.
3: under the radio config: mqtt menu, enable mqtt, set the root topic to msh/US/TX
4: click send
After it reboots, configure your channels, they need to be setup on both the gateway node and on your nodes.
You can use this my channel list here if you’d like, otherwise
You can set your own up. Unencrypted public channels use AQ== as their psk.
My channel config: