This is a new domain on an IP that was fairly recently assigned to it. All main email providers blacklist anything that is not gmail, outlook or aws by default. Nothing I can do about it, other than becoming part of the problem and paying them to host this instance’s email.
I too have tried to sign up to no avail. I just never receive any emails for verification. I’ve used both gmail and a custom domain on Google workspace (actually thinking of migrating that to mailbox.org or protonmail)
I used proton mail and had no problem signing up