Apple TV 4K cannot learn IR signals for receiver volume control

Just set up my new ATV 4K (2021). My receiver is quite old (Sunfire TGP II), so no CEC. When I try learning the sound controls from the receiver remote, the Apple TV just does not see the IR signal. At all.

I have the receiver's own remote and a universal remote, both of which have the receiver controls and some others programmed onto them. Apple TV DOES recognise those "other" signals sent from these remotes, so I'm sure the remotes are fine and functional. But it does not see the signals that control the receiver itself. I have an old Apple TV (3rd Gen) lying around, so I tested - and it can't see those specific signals either.

A web search gave no results, only a few loosely related threads with no solutions.

I have no idea what could be the reason for the problem, let alone how to fix it.

Any suggestions?

P.S. I have seen some threads where people say their Apple TV learns IR signals, but does not repeat them correctly. This is NOT my case. Mine does not SEE the signals (the progress bar in learning mode doesn't fill up, and a line saying "No signals received" comes up in a bit).