Combine a LTE/4G Router and a wwan-Router into 1 LAN

Hi everybody!

I've tried to make my home network a little more stable, but I'm stuck on the last steps.
I made a diagramm of what I'm trying to do, hope its understandable:

Router 1, a Netgear R6100, runs OpenWRT 19.07.3 r11063.
Router 2, is a ZTE 4G LTE Router, which runs stock firmware, and is not compatible with OpenWRT.

Router 1 should connect to a wwan over 2,4 Ghz Wifi, and distribute a wlan over 5 Ghz.
The APs of the wwan are faulty sometimes, so I added the 2 nearest APs to travelmate for switching. Also the 4G connection is not really fast/steady, so I want to use it just as fallback/failover.

I made an static alias interface for the travelmate dhcp interface, as travelmate needs dhcp, and MWAN3 needs static.
Afterwards I connected the Router 2 lan port (only has 1) to the wan port of Router 1, and configured a wan interface in Router 1 (IP=, Gateway=
Then I configured a "wwan before wan" rule in MWAN3.

Until this point everything works as intended.
I can ping on both "outlets" seperatly, and after some timing tuning, the handover in case of AP failure is also running nicely.

The problem I have is that, because of wwan-APs switching channels, I cant create a 2,4 GHz wlan on Router 1, as it will lock the channel.

Therefore I would like to create a 2,4Ghz-wlan on Router 2 (which runs no dhcp-Server, but can if needed), and connect it to the lan/wlan-bridge of Router 1 (in same subnet). Also I would like Router 2's Config Web Frontend to be reachable from this Subnet, but thats not so important.

I tried to add the wan port (eth0) of Router 1 to the lan/wlan bridge interface, and in this case clients connected to Router 2 get an IP from Router 1 and connect to the "big lan", and also have access to Router 1's wwan, which is exactly what i want.
But with this config, I can't use the LTE connection, as I have dont have internet access on wan interface / eth0 at Router 1.

I know this is a very complex setup for a home network, but I try to make the best out of the limited resources I have at this location.

Grateful for every help!

BR, errorclub