@sayagi1
Your options when this occurs is...
1. setup your new wifi at router to use the same login credentials as your old wifi (old network name and old password) so the RokuTV can use the last known network connection. Then you should be able to connect both your RokuTV and again use your mobile app as remote. (basically, your device is now issued new LAN IP address). After successful connection, can then change both the network and password at your new router.
2. Use a mobile hotspot similar to instructions above. Change your hotspot password and hotspot name to your old wifi credentials. If using phone as hotspot, will need a second phone to run mobile remote app.
3. Purchase a compatible RokuTV universal streaming remote. (walmart, best buy, target, etc). Just verify that is compatible explicitly with RokuTV. (This manner allows you to "infrared these TVs".)
4, Purchase an enhanced Roku Voice Remote. (also available walmart, best buy, target, etc).
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