Screen Sharing App in Mac OS 15.4 suddenly stops receiving requests

I apologize in advance for inadequate details as I am a relative novice.

I assist my 85y father (who lives 5000km away) with his iMac, using the Screen Sharing app in Mac OS 15.4. We have been using this app for over a year to allow me to control his screen.

Last week I was asking for permission to control his screen, and could hear the "Bing" of my request being delivered to his desktop (we have a phone connection to talk during the initial acceptance of my request). He was unable to find the appropriate permission box to accept my request, and the request timed out. Since that time he does not receive the requests I send through Screen Sharing.

We both have identical intel iMac 27" 2020 models, running Mac OS 15.4.

I am able to successfully send requests to other Macs running OS 15.4 (my wife's 24" iMac M2) so my iMac's ability to send seems OK.

We have rebooted both computers multiple times with no effect.

It would seem the problem is receiving the request on my father's end. We have not added any new software applications and are on the most recent version of Sequoia. His iMac's wireless internet connection is otherwise normal

I would appreciate any suggestions you may have.

Thank you all in advance.