I haven's seen one of this mentioned in a long time, but some people have reported an inability to connect if their wifi password contained non-alphanumeric characters.
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Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X
Verify with your neighbor that he does not have Family Mode Enabled or any setting on the network app that may be "Pausing" the connection to his network for the RokuTV.
The fact that you can connect this RokuTV to your wifi, but not his own while others are connecting to same network, seems to indicate that a setting (probably from the mobile app) is blocking connection to the TMobile network.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102