After logging to the KDE when first time i used to connect any network it shows the data speed but if i disconnect and again connect it it never shows it only shows obyte!! It does not matters i am using the wired connection or mobile broadband!!
Also cannot reproduce, I remember there were some issues in Plasma dataengine we use for this, maybe when you update to latest version this problem will be gone.
But how can i update ?? I am using Debian 9 !! and i am up to date!!
Can you mention which version you are talking about of plasma?
The current Plasma version is 5.10.4.
But how can i upgrade it !! i am using debian stable V9.0.1(stretch) and KDE plasma V 5.8.6 ?? Can you describe it briefly how can i upgrade it to v 5.10.4 ? !! and 1 more thing is it safe? to upgrade it to latest version?? bcz i am worried about 1 thing is may it can brake th OS/debian !!
Please ask in a forum of your distribution how to update KDE software. Usually this means installing from backport/testing repositories.
I believe that this issue has been fixed in plasma-workspace. Please go to Debian support (forum, wiki, etc.) to help you with the update.