Have you tried all the codes listed for the UR5-8400? They are 001, 011, 055, 187, 053, 112 and 205.

I couldn't find anything in the receiver manual to show that the AV1/AV2 code is present, and the front of the receiver has none of the usual buttons used when changing command modes.

You could try turning it off, then press and hold the power button and press each of the few buttons visible on the face of the receiver and see if anything happens, which I doubt.

If you have any other programmable remotes you could try making one of them work the volume on the receiver.