Route MQTT+Webinterface trough Landlords WiFi


I've got a AMS Pow-K Power Measure ioT Device. Sadly its out of Range from my WiFi. So instead of using a WiFi Repeater i tought i'd route this device over the WiFi from my Landlord.
Works fine, device is connected to his WiFi, i can access it via Smartphone.
I've even connected my Router as a Client to the WiFi.
But here comes the issue.... both use the same Subnet.
My Router IP in his Network: IPv4: on my own network it's
The device i want to connect to is on with Mac D4:F9:8D:6E:86:96

Is it possible to keep the same Subnet? Because right now i can't connect to the device from my own Lan.
How do i have to configure the Firewall that ALL Access will be blocked except Web (Port 80) and MQTT (port 1883) for this device?

For trial&error i set the newly created interface "wwan" to the same firewall zone as "lan". But i still can't connect to the device... is it a subnet issue?

Why can't you just change your network?

I got an AccessPoint on my Router. Thats my WiFi.
Then there is the WiFi from downstairs which has enough Range to reach the ioT Devices.

So the router additionally connects to the WiFi from Downstairs so i can reach the ioT device!

Yes, I think I understood that. But you have issues when you connect both networks, because they are configured on the same IP range. Why can't you just change the IP range on your network?