hello guys . i just saw another post tell us we can download Fiddler 4 solved low DL problem . so i tried it is works for me.
Instructions:
Install Fiddler 4, the software is in English, just click the button in the lower right corner.
After installation, find Fiddler 4 in the start menu and open it, then select No in any dialog box that pops up until you see the main interface.
Then quit Battle.net. Restart Battle.net.
Then you will see the domain name ending in battlenet or battlet appears in the Fiddler 4 window. If you are the same as me, you will find that Battle.net can be updated normally.
(If you see the domain name related to Battle.net displayed in red in Fiddler 4, it means that you really cannot connect to this domain name normally, you can report it to the customer-service)
hope everyone can solve the problem.
many thx CN18CK for share the solution 