Can't acces subdomain from Chrome in specific cities

The title sounds weird, I know.
For a few months I could not access my own subdomain, living at https://mp.costimihai.ro and I was simply accessing it directly via IP, but friend A, living 50km away from me could access it.

A few days ago, I asked friend B who lives in the same city as me, uses as well a macbook + chrome, and he could not access it either.

Web server is down Error code 521

Today, it finally clicked for me, and I checked with Safari, and…it works. Tried Chrome in incognito, still doesn’t work.
I tried to access it with https://www.host-tracker.com/ and it’s available from almost all countries, except for Russia and Iran.

Would anyone have any insight into what the issue could be?
Thanks

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