could it be that Pi-Hole is blocking my DLNA server internally?
I have Twonky running on my QNAP NAS but every device (TV, BR player, PC) can't find the DLNA server and vice versa - Twonky is not showing my clients (beside my smartphone). That's really strange because before Pi-Hole everythin was working fine. Sometimes - really rare - my TV is finding Twonky.
My AC87u ASUS router is running on Merlin FW 384.13_10 (maybe here a problem)?
I would remove Pi-hole from the network and return it to the 'before Pi-hole" state and compare client behaviour, and then bring Pi-hole back and compare again. This will give you a firmer ground on which to suspect Pi-hole being involved.