Yes, you are right, I didn’t check carefu enough, I am sorry. But still the problem persists;

I have Open Sans Latin-Ext for regular paragraph text and Dosis Latin-ext for headers.

– putting the @import code into the custom.css as suggested doesn’t affect the site at all.

– putting it in header.php works only for Dosis but not for Open Sans. Yet this solution is inconvenient for future updates.

– as a result in body / paragraph text (Open Sans font) the extended are not displayed.

I have practically completed the site but still can’t show it my customer because of this fonts issue.

Usually I sort it out one way or another, but on this one I’m helpless…

P.S.
I would also recommend for your future work to implement the option of selecting the cyrillic / latin-ext subsets because a lot of people are not making pages in English and it takes you more support time…