So there is a problem with your device's software, because with the option to get the time from the network disabled it should not reset the date when you connect to the internet. And I have tested it with my phone: neither connecting to WiFi nor cellular data reset a deliberately incorrect date when I had that option disabled (while turning it on corrected the date instantly).
In which case you are stuck, unless you can replace your system software with a version where that feature works correctly.