Requesting Assistance With SSL/TLS Error, Site Not Being Served Over SSL

rainbowforge.pages.dev (deployed at domain rainbowforge.dev) is my portfolio website. Recently I was notified by a potential client that they were unable to access my website because their browser is giving them an "This site can't provide a secure connection" error. I just heard back that they are able to access my site at rainbowforge.pages.dev, but not at rainbowforge.dev. I've not seen any issues myself on any of the various privacy focused browsers I use, but they sent me a screenshot of their browser, they're on the most recent version of google chrome (which I even bloody downloaded to check, and still couldn't reproduce the error myself), saying "rainbowforge.dev sent an invalid response | ERR_SSL_Protocol_Error" (screenshot sent by potential client included). After receiving this email, I looked around in the dashboard and found that my website is being served to between 5-15% of visitors with no SSL. I'm unsure as to why this is, or how to fix it. If anyone has any advice for me, I'd really appreciate it! Steps I've taken already: I enabled automatic HTTPS redirects through the universal SSL dashboard. I'm not sure if turning on HSTS would help or make things worse in this scenario. Thanks so much!