Not fast - Not reliable

If you have the mobile application on your phone get the cellular metrics from the gateway and post them to the conversation. Maybe there is a way to get a better signal but knowing the RSRP, RSRQ, and SINR values for the 4G and 5G NR signals can be helpful. The positioning with the LED and the bars does not tell you enough about both the 4G LTE and 5G NR signals. If you can improve the 5G NR signal strength and quality it might just hold. 

http://192.168.12.1/TMI/v1/gateway?get=all <-To also get cellular metrics from the GW.

You would also benefit from knowing where the tower is and how far from you it is. Getting the gateway so it can receive the signal better may be possible without and external antenna. If you do have a good path to the tower without too many trees, hills, buildings, etc… degrading the signal then an external antenna can be helpful. Use the cellmapper.net website to find the PCI, physical cell identifier for the 4G and 5G signals. The cellular metrics should provide the PCI value and with that you should be able to locate the tower. If you have an n71 signal a 2X2 MIMO antenna is sufficient and a vendor like Waveform.com can help you understand what could possibly work for you and provide information about setup. 

If the T-Mobile solution just does not reach where you are maybe Verizon or AT&T have service that could reach you but then contract and fees are a downside to that picture. If you post more information about the operation of your gateway and the cellular metrics users on the community can offer more suggestions. You might just get it stable and get what you want.