as you mention "old 5406zl" - I do assume those are just providing PoE (and not PoE+) power.
do you have IPM enabled on the IAP-515 ?
if not - pls. enable this, it might help.
if yes - you need to provide more infos...
typically if you don't need the USB-port and 2nd ethernet-port - those can be switched off without any issue and that might be sufficient already.
regarding PoE(+) configuration on those older switches there does exist a good guide: https://www.hpe.com/psnow/doc/c03597282
note: I cannot promis that just enabling IPM will fix your issue, but power-problems can cause strange behaviour.....
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