Any way to disable 5G connection on router?

Sorry for the delay in response. Apple mail decided the message was junk mail. 😒 From the stats you have in your message it is pretty clear the SNR is very low. The SNR at 4-5 dB is on the low Medium part of the scale. That is going to impact performance. The RSRP in the -99 dBm also places signal strength in the Medium range. At least it is at the upper part of medium vs. the bottom. With those values and the distance to the tower it would be interesting to know bands the router locks onto, the  (NR) band n41 (2.5 GHz) or the  n71 (600 MHz). If you are in a more dense urban area with n41 on the 5G that would explain the low signal strength. The low SNR could be due to the distance and other factors such as buildings, trees, hills, RF noise from various other sources. Even the LTE signal has a low SNR so there are some external influences possibly. With the PCI values the tower location and knowledge of the topography in between can explain a good deal. You stated, "our usual 400-500mps 5G speeds" so I am going to speculate that it is a n41 signal source. Based upon the research i have done reception of n41 channels would be best dentro a 3 mile radius. Outside that it might be sketchy. The closer the tower the greater the speed and the noise would be possibly reduced. It all depends upon the other factors in the area. The shorter wave length of the n41 also impacts penetration so outside a 3 mile radius with a marginal signal is a challenge. Since the RSRP is still in the medium range an external antenna might make a reasonable difference. I looked into the waveform.com solutions when the 5G here was flakey. It is an investment as it is recommended to have lighting arrestors as well as the antenna so it pushes the $400 mark for even the lesser external antenna solution. For my solution  the 5G issues were related to work on the tower so I backed off the external antenna idea. I receive pretty solid signaling for both 4G and 5G with very good SNR and i have line of sight to the tower 5.3 miles away with the n71 NR signaling. It works pretty solid though the speeds with n71 do not compare to the n41 but well a solid stable solution is better than a flakey fast one in my opinion. 

If there are other towers in the area and T-Mobile can provide greater signal saturation from another tower to the location that is a direction to investigate. It does depend on your location. Here I have only one option as there are lots of pastures between us and denser population where the towers tend to be.