"Certum Domain Validation CA SHA2" is an intermediate cert, that chains
up to the "Certum Trusted Network CA" root cert that is included in NSS.
NSS includes root certs / trust anchors (so not usually intermediate
Websites are expected to serve up the intermediate cert(s) along with
their TLS cert. The site https://lk.peterburgregiongaz.ru/
serving up the TLS cert.
Other than that, I do not see any problem with the TLS cert:
And the intermediate cert looks fine too -- just needs to be served up
by the webserver.
Hope that helps.