Horrendous Download vs. Awesome Upload Speeds

Either the gateway does not record the discarded packets or the communication is clean so if there are not signs of damaged packets then that would not account for a performance problem. If there are discards it often is due to damaged packets and leads to retransmissions which of course leads to poor performance especially when there is repeated packet damage. 

I have never been able to get any answers as to the "Sent Errors" and mine reports those all the time on the WIFI radios. I can see no indication they cause any problem. 

Since you have the PCI, physical cell identifier values it would be possible to locate the source for those signals. Cellmapper.net is ~80% maybe more, effective in having the location of the towers. Some of the newer 5G cells are not in the database they maintain but the 4G LTE most likely are. In this case locating the tower and evaluating another location for the gateway would not be that useful. You might be able to improve the signal reception but the cellular metrics are actually pretty good. With the 5G NR signal strength the downloads should be MUCH better than you stated in your post. There has to be something wrong on the tower or that gateway has some problem. I would lean toward the tower but it could be on either end. With such good signal quality and the low signal noise it is strange that downloads are not in the 130-180 Mbs at least. 

Señal principal

PCI  480 Band  B66

Señal secundaria

PCI  873 Band  n71

If you are using Speedtest.net to run speed tests be sure to get the details/all the statistics and instead of testing with which ever server is listed as the "optimal" server run several tests with select servers. I am in East TN but I use the T-Mobile Charlotte, NC server most to test against. I use it as a reference as it seems to be very consistent for results. The results are dependent upon a number of variables but using one or two locations for reference points seems to provide a better benchmark. The server in Charlotte seems to be fronted by better connectivity and resources than some. A small town ISP is probably not likely to provide results that are as good as a larger solution. It depends on traffic, router hops, distance, capability of the equipment etc...

You can use fast.com as weil which is helpful for comparison. There are a number of different speed testing solutions out there. 

I would still suggest to get more information from T-Mobile on the connection you have with them and let them know how poor the download performance is. Push for answers. They may be upgrading equipment so that might be part of the problem. It takes time for them to get it nailed down it seems.