how to get back to “normal” speeds?

It would be good to know where the tower is your signals come from. I speculate T-Mobile may be making upgrades in the area and that has resulted in the gateway's loss off signal. You can call T-Mobile support and ask for locations of towers around your home and what bands are broadcast off the towers. If you want you can also go to cellmapper.net and look for T-Mobile 4G LTE and 5G NR towers in your area. CellMapper.net is purportedly around 80% accurate and relies upon subscriber data for updates to the database. It is actually one of the best sources to use for tower location. You can filter by vendor, signals 4G LTE and 5G NR and even down to the bands. I find it most helpful to not filter down at first to the bands as if you filter to the mid-band n41 you will not see the lower band n71 and you might be deceived in believing there are no 5G towers in the area. The 4G LTE will show you a pretty good representation of where the towers are. The 5G filter will narrow the results down a lot. In some cases the actual 5G that IS there may not be in the cellmapper.net database so it will not be visible on the map with other towers. At least that will give you some idea where the towers are and what signals are from each. That is a place to start. With more understanding of the router metrics for the cellular signals and the PCI physical cell identifier from your Nokia gateway you should be able to identify where the gateway does see a signal possibly. If your gateway does not see a signal at all then calling T-Mobile and asking them may be the only option.