Weird issue on connection PC->server when browsing from smartphone

Hello,

really, I can't find the culprit here and ask for help.

1) I do the login from the PC browser (eg. Google Chrome | Win 10 Pro 64bit) with superuser or another user both.

No problems at all: I can surf the cloud and the PC to server connection is normal as usual for any other action, visiting the website, the server control panel and so on... 

2) I do the login from any smartphone with any browser...here something weird happens but on the PC

The internet connection from PC to the web server is not functional anymore. I checked the logs on the server and I can't find a reason or a specific error. What's the difference, please? Maybe my router which does strange things when accessing the cloud from smartphone?

p.s.: the problem is only about the web server and FTP, eg. I can access the server on SSH from the PC as usual but can't connect to https or ftp...while on the smartphone ALL is working

Thanks for your help