Are you trying to say that integrations aren’t being discovered because you have multiple LAN networks?
If so go here and find the integration for the thing you’re trying to add. Then follow the directions. Discovery is just convenience, you can add things manually even if they weren’t discovered due to complex network setups or whatever.
If that’s not what you meant then please clarify. There are many different types of discovery in HA and your post is extremely vague. Please explain your setup, what you’re trying to add that’s not being discovered, why you think that is, what you’ve tried, etc. I.e. provide the context that would help some one help you
And also leave out things that aren’t relevant or explain why they’re relevant. Not really sure what MQTT has to do with anything here.