Not fast - Not reliable

Re: 5G numbers. RSRP -105, RSRQ -12, SINR 8. LTE -99, -13, 13.

Sure would be happy to try to help you out. With the table for the cellular metrics values I can see right off the 5G signal the GW is reporting is very weak. The RSRP is the signal receive power, the RSRQ is a reflection of the quality of the signal, and the SINR reflects noise level for the signal. The 4G LTE is in the medium range for strength and the RSRQ and SINR are in the good range so though the 4G signal is not good or excellent range at least the quality of that signal is not terrible. With the 5G values I can understand how you would have, "lots of trouble”. It could be the distance to the tower and/or a combination of external factors that degrade the signal. 

If you can find out where the tower is that delivers the 5G signal it could really help to improve the positioning of the gateway to try to get a better reception of the signals. There are so many different factors that can reduce the signal. If you have metal screens, a stucco house, i.e. with the wire reinforcement for the stucco, large trees or structures between your home and the signal source, and well a number of other things that lead to signal disruption. Every location has a unique set of factors. 

Do you have the Nokia GW or the Arcadyan GW? The Nokia is the cylinder shaped router and the Arcadyan is more of a square design with rounded corners and an LED screen on the front vs the small LED screen on the top. The LED reporting is a very generic display so knowing and understanding the cellular metrics helps to improve the signal. The software interface for the Nokia via a web browser is more informative than the Arcadyan GW so how much information is available and the ease of access depends upon the model of the gateway you have.

If your GW is the Nokia then using the web interface via the 192.168.12.1 gateway address is a friendly way to get more info. With the web interface you can obtain the PCI, physical cell identifiers, for the 4G and 5G cells and use cellmapper.net to find the tower. With a combination of the database info from cellmapper.net and google earth maps looking at the terrain between the home and tower can help some to understand what external factors may be involved. If you have trees and hills and the tower location is beyond them that is a condition to consider. The distance to the tower also is very helpful to know as it helps set expectations for the signal delivery. If you are on the edge of the cell delivery then the values your gateway report could be explained, in part, due to distance to the tower. If you need pointers as to how to use cellmapper.net just ask questions. Cell Mapper is roughly 80% complete information. It relies upon user content delivered to the server to get confirmed tower information into the database. Many people have contributed data to the servers but it is not 100%. Many 4G LTE towers will easily be found via the PCI cell values but in some cases with more recent 5G deployments the data to show where that tower is that delivers that cell signal may not yet be recorded. Using cellmapper.net via the browser is not hard. Using the filters for 4G LTE or 5G NR towers/signals rather than no filter really helps get a cleaner reporting on the map to reduce clutter and find what you want. 

If you cant get the information from say cellmapper.net then T-Mobile support engineers "should" be able to provide more information about tower location. Getting your gateway situated with the best exposure to the source signals is very important. It can be a bit confusing at times as the signal can be direct or reflected. In some cases placement in a room in front of the window may not yield the best reception. You want to avoid putting the GW direct in a window where it would get too much heat from the sun through the glass. Taking the GW on a walk about reviewing the LED values as a starting point and then also reviewing the metrics to see detail should help. If you cant improve over 2 bars on the LED then it might be possible to use an external antenna solution but depending upon your ability to understand the process or lack of assistance from someone who does can make that a challenge. There is good information about external antennas but first see if you can just improve the reception by testing different locations for the gateway placement. Maybe things have changed since you placed the gateway when you got it and another location in the home is better.  I can't say much more without having a better understanding of the situation.