New Airport Express / Roku M500 M1001 Problem

I bought an Apple Airport Extreme router to replace my dead Linksys router. The M500 and the M1001 both worked great on the "old" router using WEP and a 26 character code (mine is all numbers).

Here is my problem when using the new router:
If I have encryption set to None on the new router, both the M500 and the M1001 work perfectly.
When I try to set up WEP encryption on the new router, I can only enter a 13 character code, not a 26 character code (still using all numbers).
When I enter the 13 character code on the M500, it works perfectly.
When I enter the 13 character code on the M1001, it can't connect to the router.

So for now I have Encryption set to None, but I sure would like to have my network secure.

Any ideas? Brad