This guide will let you know how you can fix the Connection failed after 10 retries error in less than one minute in Left 4 Dead 2.
First, you will need to go to your Steam library and right-click on Left 4 Dead 2 and then properties.
After that, you need to tap on Set Launch Options.
Type in “+ clientport 270XX”. Replace the XX with any number from 06 to 30.
And then click the OK button.