First thing to try.
Be sure to be connected to reliable Wi-Fi.
Top right 3 dot menu on the Settings > APN page > Tap Restore to default.
Power down the phone and then restart while connected to Wi-Fi. Then try to reconnect to mobile data.
Can you find SKT published APN. Check that the APN is an exact match with your plan, both within the detailed APN data and also in the
“Plan name” which you select 
One can try to Edit and Save any incorrect APN data. If it fails to Save it is sometimes necessary to use the + Add button top right to copy out the complete APN page. Be sure to return and Select the new page.
(The APN could possibly have been updated in some small way by the carrier and the AOSP “presets” not updated. This could be corrected as Wrong APN settings Telia Norway)