Why my phone sometimes can't connect to internet?

Details: Lenovo K5 Pro, Global ROM (in EU), Android 8.1.
I have a problem with data connection, when screen is locked or recently unlocked, data connection sometimes isn't working, even though the connection shows as working.
In Chrome I get errors like: err_connection_closed, err_ssl_version_interference, dns_probe_finished_no_internet.
The weird thing is, when I have turned on hotspot and use internet on my laptop, it never loses connection for the whole evening.
All apps have 'unrestricted data access' and 'background data' enabled.

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