is-a.dev is one of those community dns projects where you can ask them to give you a subdomain: https://github.com/is-a-dev/register the domain is on namecheap/not on his acct
I thought is-a.dev were now providing hosting facilities, then I gave all the files there but it didnt work (maybe i did something wrong) then I thought that maybe its because of the pages, so I deleted it from there, but it still didnt work, so I readded it
Interesting, is-a.dev seems a bit hacky as they're not part of the PSL (Public Suffix List - it would make providers like Cloudflare view each subdomain of is-a.dev as a seperate site rather then a singular), and a quick look at their open github issues shows them talking about issues with Let's Encrypt ratelimiting them, possibility related
could try deleting it the custom domain, waiting ~30 minutes, readding it, and seeing if that gives it a kick to fix itself. You could also try making a post on the community forums about it, where there is a few CF Pages Team members who could take a look. I wouldn't be surprised if it was related to them not being on the psl though, some limit on certs or something, always better to get your own website if you can