Horrible signal to noise ratio

From looking at the small window you have the router situated in with the metal frame it could be you are getting multipath propagation due to radio wave diffraction. This would be difficult or impossible to prove without equipment to analyze the signaling. I am just speculating. The router may be directly facing the tower where the signals come from but if the small window results in interference resulting in the diminished SNR. The RSRP and RSRQ values look pretty good but the noise is probably resulting in some packet damage and diminished speed due to retransmissions. It could be a capacity issue or backhaul issue but T-Mobile engineering should be able to determine the actual loads on the equipment. 

Using the external "router" for WIFI and disabling the WIFI in the Nokia router might help a bit but if the cellular signals are suffering from external influences that might only improve local wireless connectivity. If you do indeed have line of sight to the tower then a YAGI external MIMO antenna directed right at the tower would probably help reduce noise and improve performance. Instead of locating an external antenna on the exterior wall directly facing the tower it might be necessary to leverage other building surfaces to shield the antenna from unwanted signals to allow the antenna to have cleaner signal wash from the desired tower. The downside of an external antenna is the added cost. With lightning arrestors and a 2x2 MIMO external YAGI antenna you would be looking at $300-$375 to do that.