AFAIAAO, some TP-Link routers do not allow to use private IP addresses as an upstream resolver.
Having your clients talk to your Pi-hole directly would be the preferred configuration anyway, i.e you should configure your TP-Link to distribute your Pi-hole machine's IPv4 via as local DNS resolver via DHCP instead of its own, see e.g. Can't setup network-wide DNS because of router - #8 by Bucking_Horn.