In these circumstances, you can’t get back to the Stick from the App - you need the physical remote.
What might work is changing the password back to what it was before, on the router, which should let the Stick connect. Then also change the password back on whatever device the App is on, so it can connect also. With the two devices now connected, change the password on the Stick to the new value. This may lose the Stick immediately, but no matter.
Now restore the new password on the router, and on the device with the App on it. The Stick should reconnect, and the App should be able to control it.
How did you lose the remote? Have you had the cushions out on the sofa? It might, certainly for the future, be best to have a hard think about where the remote might be, and look for there.
Failing that, the best and cheapest way to get a new remote is to buy a new Stick, as the only replacement remote on Amazon that has the new smart functionality actually costs more than a new Stick, especially one that comes with a pass or passes on it.
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