I had the same problem. It could be that the certificate is rejected because it's valid for 10 years. A certificate can't be valid for more than 39 months to be validated by your browser. I solved it by following the original instructions for pixelserv certificates which creates a certificate with the shorter validity. Here is the command to create it from the pixelserv website:
Install certificates for Router WebGUI to JFFS (needs to be renewed after JFFS format):
sh -c "$(wget -qO -
https://kazoo.ga/pixelserv-tls/config-webgui.sh)"
Follow the instructions and install the new certificate on your browser. I also use the same certificate for the router UI by loading it in the DDNS section of the router:
View attachment 29475