Yes I'm facing the same issue actually I'm regretting on buying Samsung s21 fe. I am also facing the same network issue the 4g mobile data breaking at frequent intervals,its not an issue with a particular sim. I'm facing it in all sim card.Definitely...

4months

Yes..I changed the video quality settings in youtube, from high quality to auto

Yes

As a user of samsung s21fe i would say don't go for that...Both the battery life and heating issue bother me a lot....