Based on you screenshot from the NPM Dashboard there seems to be something wrong. In the setup window you show that you forward the traffic with http and port 80, in the dashboard screenshot you forward the traffic with https and port 80.

Just skip http and self signed certificates all together. Modern Browsers make it a pain to use non https sites. A simple domain setup with dns acme challenge is a little bit of a hassle but worth the hour(s) of invested time. Especially with npm were it is a set and forget option.

Does pihole support wildcard dns entries yet? To my knowledge the gui only supports single entries so that you have to enter every subdomain manually in pihole that you want to have forwarded. Workaround would be to use a dnsmasq config file or use something else like addguard.