Any way to disable 5G connection on router?

I would say from experience that all T-Mobile support engineers are not equal but if we treat them as we would want to be treated they actually do try to help out. I have had some that were OK and some that were pretty good. I kept calling back until I got answers. It is the luck of the call. Ask the questions as the only dumb one is the one not asked. Give them data if you can. Details help support. Ask the support engineer to tell you what the coordinates of the tower are so you know where it is. Get the PCI value for the secondary signal from the web interface and use cellmapper.net to see if you can find the 5G cell with that PCI value then you should be able to know exactly where the tower is. Keep in mind that cellmapper.net is only 70-80% accurate. T-Mobile knows what tower the router connects to. 

If you document the time(s) of the issues with the 5G cellular signal then T-MO support can probably look at maintenance records for the tower and see if there are radio monkeys messing around. When there was work on the tower equipment here back in June I saw times where the 5G, secondary cellular signal would drop. The transition to the 4G LTE signal would wobble and a disruption would take place. It was very frustrating. As soon as the 5G bounced back it could all go wonky again. It was very frustrating over and over. You can't just disable the 4G LTE or 5G cellular radios on the router. Share your observations with T-Mobile support. If more users on a given tower all did so they would know faster that equipment on a given tower was acting up. I would guess their monitoring equipment would inform them as such, but if they are doing upgrades and fine tuning the tower equipment that might be a good place to start. They should be aware of work on towers.

If you have a 5G phone you can leverage applications to use a different device to look at the cellular signaling. There are applications for Android phones that are pretty good. On Apple iPhones it is more limiting due to Apple's security policies. If you have an iPhone you can put it in field test mode and look for cellular signal information. Dial *3001#12345#* then enter. You are in field test mode to see what the phone knows about the cellular signals. It takes focus to see what you want but it works.

It could be a problem with the tower or the router. Knowing if the tower is being worked on or the equipment has been acting up can help. If the tower logs and statistics do not reflect problems then maybe it is the router itself and not the 5G from the tower. Paying for 5G and only using 4G LTE is a poor workaround.