AP Isolation

The 12.200 was an example. Please notice the >>192.168.12.200 <rtr interface? I am assuming you are using the ASUS to route to the 192.168.2.x network from the 192.168.12.x network.  You state "From router .2". It is not clear to me what you are doing. You need a router with two interfaces which has one in the 192.168.2.x/24 network and the other in the 192.168.12.x/24 network. A router with directly connected networks will be aware of both. The T-Mobile router has no way to know or be told that the other network exist without admin rights to provide the routing. If we could put a static route on the T-Mobile router to that network or use dynamic routing and run a routing protocol it would be possible but we don't have that luxury. Below ( 2 routers, yours with 2 interfaces & the Nokia with 1)

Is this what you have in effect? This is what you need. A router to pass traffic back and forth. The Nokia router will only ever send traffic to/from the GW interface between the local LAN and Internet.

192.168.2.x/24 NET--Rtr INT- Routing Device -Rtr INT--192.168.12.x/24--GW INT<192.168.12.1>

 Otra red               ^                   +                   ^     T-Mobile default network       +       ^

                             (192.168.2.1)    Router B    (192.168.12.???)  -------------------- GW router A

I am just using the bold print and underline etc… for emphasis and hopefully clarity. Again you must have the route statements on clients on the 192.168.12.x/24 network so they know what router to send the traffic they have destined for the 192.168.2.x/24 network segment.