Sorry but your biggest issue here is not clearly stating your whole network layout first - you keep adding or modify bits so it is like trying to hit a moving target. Like Windows 10 suddenly 'appearing'.
You are just jumping from one bright idea to another without ever pinning down and solving an issue.
It is just a xyinfo problem as I have previously suggested.
Your issue is completely one of routing and your network setup.
It WILL work if you have set your network up correctly. You do NOT need anything else - no extra swtiches or VPNs.
I think as has been suggested to you at least twice now that you need to look at your main router - your Fritzbox.
That needs to know where some of your networks live.
Then when each local one does not know where to find another network, it can query the main router for directions.
I really don't know what to suggest to you right now.
I guess go right back to basics and get two Local Networks to talk to one another as per your early diagrams where you were trying to get a 192.168.1.x net to talk to a 192.168.2.x net
Revisit that and start again. Make sure your Fritz box knows where those networks live. So it probably needs a route like '192.168.2.x can be reached via 10.0.3.1' or similar, and the same in reverse.