Video Calls on Messenger do not ring on iPad?

My 80 year old mother, who is in a different country from me, has a problem with her relatively new iPad in that when I try to video call her on Messenger it does not ring in anyway to alert her that I am trying to connect in this fashion. We have tried every suggestion I can find on this community platform but without success.

Could somebody please fully consider and then share the exact settings which would allow this fairly normal function to work?

Please remember this is an iPad not a phone.