If you are talking about the auto-upload feature, there were problems with permissions that Google did not grant. But that has been fixed and at the moment it works for me.
See also here: Unhappy with the recently lost file upload feature in the Nextcloud app for Android? So are we. Let us explain
Regardless of this, you need a new device because your device doesn’t receive any security updates anymore. Also, eventually more and more apps will cause problems or stop working at all because they will no longer be compatible.
And no, you can’t send the bill for the new device to Nextcloud, regardless of whether it works the way you expect it to or not 