Can't use my own SSL certificate for Portainer?

I set up the Nginx proxy manager container to avoid the "not secure" warnings on my internal Docker containers. They all seem to work with my duck.dns SSL certificates and I can access them using custom domain names (e.g. Plex.duck.dns.org) but when I try it with Portainer, I get this error on Chrome. Firefox and other browsers don't show the error but tell me that my connection is still not secure. Does anyone here know why and how I could fix it?

https://preview.redd.it/7rsxc10mh60c1.png?width=897&format=png&auto=webp&s=d0532cef522c6b7fb2c482c8d946b9a0bb7fd6a0