Some websites and software cannot connect to the Internet

Hello, as mentioned in the subject line, I encountered this matter on Monday.
Overall, everything works fine before this Monday. However, several websites, including the trading app on my laptop from those websites, suddenly can not connect to internet. 
I used All my family devices (laptops & cell phones) to test, but cannot use those websites/applications still not connecting through my home Xfinity WIFI or public Xfinity WIFI, but those websites/applications works when we use our cellphone hotspots ( T-Mobile & Version). And, I also bought a new modem to test, problem didn't solved. 
I've chat/called customer services several times... I really need this problem get fixed...
The reply to my last request says  I need to contact website owner, but if website/application works under other WIFI, I guess the problem is from Xfinity.

Just couple minutes I tested ago, a website and application successfully connected for couple minutes but super slow, and disconnected again. 

Could anybody help fix this issue? Thanks!

Here is the link https://www.tdx.com.cn/ , and the application (stock trading application, it's in Chinese name) is provided by this website