You should address this question to your cellular carrier. HMD Global filters access to OTA updates by carrier authorization. Either your carrier is slow on testing and authorization of OTA updates or it does not sell/support Nokia handsets at all. If latter is the case you'll never receive OTA updates. You have two options: 1) insert SIM of an appropriated carrier and check for updates; 2) install updates manually, you can google Nokia 6 OTA zip files on dev forums. Though note that monthly OTA updates are incremental rather than cumulative and should be applied one by one in chronological order. If you stuck in April you will need May, June, July, August, September, October OTA zip files. Updates for TA-1021, TA-1025, TA-1033, TA-1039 are the same.
Does not matter whether your phone SIM-locked or not. The carrier you're using should authorize OTA update.