CAs issue certificates for domains. IF (big IF) you want a CA to include IP then you have to prove you OWN and CONTROL that IP (think Cloudflare 18.104.22.168). So no, you will not get a commercial CA to issue a cert for your fixed IP. You need one for your domain, free or paid.
ASUS DDNS should work for you. Some ASUS routers have built-in provisioning and renewal of LetsEncrypt certs. If yours does then you can follow this option:
If your router does not have LetsEncrypt support then you have to follow the normal paths to get the cert.