To fix your old domain name showing the NOT SECURE error message, I would require your web hosting provider for the old domain name to have support for apache or you would need to purchase a SSL certificate for your old domain name.
As for the problem with the .com/XXXX pages, I would need a copy of the entire website to ensure there are no missing files on the web host. If there are no missing files, I would check to make sure the domain name references are set up properly.
Note that I would need access to your web hosting provider and domain name administration. I advise you to change to temporary passwords, then change them back after I am done fixing.
Previously, I have experience in migrating websites to a different domain or web hosting provider. I would be very happy to assist you with your technical issues.