Hi,
Thank you for posting your query. According to your description, here are some suggestions and an official document you can refer to.
Please check if you have added the remote PC on your Microsoft Remote Desktop app?
Open the app and accept the agreement. Tap the + icon to add a connection and select the Desktop option.
If you are near the remote PC, scan for it wirelessly. Or you can add it manually by typing the PC name and surname then Save the changes.
Remote Desktop - Allow access to your PC from outside your PC's network
https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/remote-desktop-allow-outside-access
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