Please help! My zoom students from China can no longer access my PM on zoom why?

My zoom students from China can no longer access my PM on zoom why? I have been using the same pm room for several students since 2019, everyone has the newest version, I did not change any settings all of them has VPN (virtual private networks) so it can not be blocked. HOWEVER in one day, all of them could not longer access my PM room why? 😞

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