tadiwa51 wrote:
The cellular network is experiencing frequent connectivity drops and poor performance. after the update
But you said a day after the update. If the update caused the problem it would have been immediately apparent.
The first step is to call your carrier, and ask if they have a local outage. You can also check up on them→Down Detector - Crowdsourced Outage Check
Next:
Do you have a 3rd party VPN installed, even if you are not using it? If so, delete the VPN profile in Settings/General/VPN & Device Management/VPN, then restart your device. Don't just turn it off; delete the profile. If that fixes it, you can try reinstalling VPN, but probably shouldn’t. Any update can break a VPN until it is deleted and re-installed.