I don't know your location, but assuming there is a Walmart or similar store in your area, try any remote you like since it can easily be returned.

If you need this much remote, the Sony RM-VL600 is a good one. Walmart used to carry the OneForAll remotes, and I recommend them, but if you can't find them then the Sony is the next good bet, and if they have a cheaper non-learning Sony you could try that also.

From what I've learned so far, none of the RM-AAU series remotes have the capability to change command modes on the remote, and if you can't change it on the receiver then it may be set at AV2, which is a bummer.

Do you have any other equipment (VCR, etc) that has a code listed in the UR5-8400 manual? If so, you could see if the UR5-8400 will control it, which would tell us a little more about this problem.