Do a backup and factory reset it after os update like major update it will be like this. I have used mi a1 stock Android it also had same issue after major update battery will be reduced and performance also. Once reset it should work.

We have 6 Samsung phones at my plave out of 6 we updated 3 in August and they started having this issue. Other 2 are working fine and one got update now that fixed this bug

There is a software bug for most of the Samsung devices. August patch update started this issue and they are rolling updates to some phones fixing the issue.

Network fluctuatea like from full to 0 and again full within minutes

Try App2Sd i don't if it works now but we used to do it with this app. https://play.google.com/store/apps/details?id=com.a0soft.gphone.app2sd