Having a backup of your HTC EVO's WiMAX RSA keys may give you some peace of mind, but it won't do you a whole lot of good if missing/broken keys aren't the reason your 4G connection isn't working properly.
That's why xda-developers member strung strongly suggests that you backup not just the keys, but your entire WiMAX partition.
He's shared a simple command, which you'll find in the link below, that you can input into a terminal emulator on your phone (much like you may have done to backup the keys) or in ADB on your computer that will create the backup for you. Note that this will only create the backup (a file called wimax.img), not restore it.
A more "automatic" approach to backing up and restoring your WiMAX partition is to simply create a Nandroid backup using recent versions of either Clockworkmod (v22.214.171.124) or Amon_RA (v126.96.36.199 and up). Simple as that. There are no extra steps involved to backup or restore it; just do both like you normally do and the WiMAX partition will be included. And since your EVO is already well acquainted with making Nandroid backups (tutorial here), there's no need for another how-to.[xda-developers]