You didn't answer the question about whether the two PCs are on the same LAN behind the same firewall, I would guess yes, and I would guess it's the firewalls problem. rebooting it might help.

Hello chesty,

No, the two PCs are not in the same LAN, and are from different ISPs. (One is my PC at home, one is my working computer in the office)

I remember I could use my mobile phone to ssh to the server within the WIFI at home. But as I tried today in the office, the mobile phone could not connect within the office's WIFI, either. While the mobile phone could still ssh to the server with mobile data connection (A different ISP with the office's, also different with my home's)

While listing these out, I think it is more likely the problem is on the ISP side...(Especially with the behaviour of my mobile phone)

I'm a resident in China, I guess there may be some "upgrading" of the China's GFW...

oh, you never mentioned you were in china, china blocks ssh and vpns all the time, it's likely the new port will stop working soon.
and now this forum has the word china in it, it will likely be blocked soon too.

Well, I hate to say this, but you may be right...

Thank you very much, chesty, to help me tracing this...

I think I need to find some alternative solutions to keep myself unblocked...