Date and time are set correctly on the iPhone.
No VPN or proxy (e.g. via adblocker) is used.
Both WLan and cellular work otherwise - e.g. Safari opens Apple.com.
Your settings allow you to connect to the App Store and install, update, or purchase apps, location is on.
The App Store servers are up and running, your cleared the App Store cache and have the network and cellular connections resetted - all
as recommended by
@russell_314.
Well then,
turn off Wifi and Bluetooth, put the iPhone in airplane mode.
open
Settings>General>Date & Time. Turn automatic date and time setting off and set the data manually to one year in the future, e.g. 20th of july 2024. Wait 5 minutes.
Restart the iPhone.
Turn Wifi and Bluetooth on. Keep cellular still off/air plane mode on. Set the date manually to one week in the future, e.g. 28th of july 2023. Wait 5 minutes.
Restart.
Turn on Wifi, Bluetooth, but keep airplane mode still on.
Set the DNS to 1.1.1.1 or 1.1.1.2.
Set your date to automatic and wait till the correct date is set. Open the App Store. If you get the “cant connect”-error, delete the App Store cache (tap rapidly 10 times on one of the lower 5 icons). Close the App Store. Turn off airplane mode. Reopen App Store.