Most likely just an interference issue. Difficult to say offhand whether the booster is causing interference at the Roku or the router. Seems some people cannot find their 5ghz networks at all if boosters are adjusted too high or when using router channels 153 and above. (not specific to Roku devices, just in general online complaints of TMobile boosters)
One option is to change the router's 5ghz channels to the lower frequency of 36 or 48. Can also see if performance increases if you use the Roku devices on your 2.4ghz band rather than the 5ghz. Since you said "not too tech savvy", other option is to lower the boost strength of the T-mobile booster (adjust its settings), or move the booster farther from the router or the Roku devices themselves. (May have to experiment a bit).
The other least helpful tip I can offer is to of course unplug the booster when streaming and use when necessary. Hopefully one of these will help you and post back if you need any more help or you find a better solution to your problem.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
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