I have restarted all network configuration. Tested it and it did not help - there is still the same problem - I am going to elevator - going out - internet stops working - changing between preferred network type (or phone restart) - and it is back again.
This doesn't look like a firmware issue, but a modem issue.
You can try flashing different modem via twrp that fixes most issues (compatible with all sdm630 models: s2 4/64, c10, s3): https://4pda.ru/forum/index.php?s=&showtopic=892500&view=findpost&p=94483328
After flashing, it's recommended to reset network settings: Settings -> System -> Restore Settings -> Restore Wi-Fi, Mobile Network and Bluetooth settings.
Also, it could be your mobile operator config issue, not re-enabling mobile data for your after you lost signal and not related to device at all. Changing network LTE/3g effectively re-registers you to that operator thus re-enabling mobile data.