How do I stay on n41 band on my trash can?

Have you identified the PCI for the n41 and n71 channels and located the tower(s) that broadcast on these? The signal change might be due to signal strength due to the proximity of your location with respect to each source. I can't say with any certainty but there might be some relationship there. The n41 will not be able to travel as far as the longer n71 signaling so it might be if the n41 signal is lost and the n71 is available it then transitions. You might be able to locate the tower(s) with cellmapper.net and get a better feel for what is taking place. It would probably be worth the time to talk to T-Mobile support and explain how your router is acting and find out if there is work being done on the tower(s) in the area. They also could provide coordinates for the location(s) of the two signals. If your router is on the fringe of the n41 signal it might just not be able to hold it and then transitions to the n71 source. Having the metrics for each of the signals would possibly help. Knowing tower locations and distance to the source(s) would also help. If you have had it working for some time now and this is a new behavior it might be that T-Mobile engineers are working on equipment in the area. A call to find out could confirm that.

As users of the cellular solution we have few "hacks" available to us. The best defense is to know what you have. Once you have a better picture of solution maybe it will be easier to get it stable.