I'm considering that it can be an issue with airport express IPv6 but not sure, might be another unrelated thing which then I would like to know more about.
To put it simple I'll say the following: When I'm connected direct to university network then I can access youtube but if I put airport express in between then I can't access youtube. I mean I setup apple airport express to connect via dhcp to university network and connect through it to university network.
I noticed differences in macOS network panel, here its connected directly:
And here its connected via airport:
Probably I should mention that I'm experiencing this in China, where youtube is blocked but for some unknown reason it works in the situation exposed before.
Anybody can explain what's going on?