There are couple of bugs, but I was able to connect it via bridge mode. First of all, check if you're getting internet connection when using NAT.
If that works, change MOBILE INTERFACE connection from NAT to bridge and write MAC address of your router's WAN port into that field.
Next, in LAN INTERFACE settings:
IP Address: 192.168.1.1
IPv4 Netmask: 255.255.255.0
Enable DHCP: Enable
You can use different IP, but last octet should be same for start and end of dhcp in order not to work for local addresses. You can also use different Lease time. In this case, your router behind RUTX device won't get any private IP from RUTX, only IP (might be private or public) from ISP. Also, it does make a change to use it in bridge mode as I'm getting similiar or same download speed, but 2x more upload speed - from 20mbit/s (NAT) to 40mbit/s (bridge).