Nav 2017 Web Client/APP - Certificate Error

I've been trying to set up a web client to work with the Android NAV app and through web, and the error I've been getting is that the certification SSL key is incompatible. But first, I have been able to log through other PCs within the domain, and although it works, the https is crossed out red in color and says it is not secure. , but outside of the domain, it doesn't even get to the authentication despite having a public IP. Our system administrator has checked both the firewall and the ports on the router, and apparently there's no issue there. We're using Windows authentication for Navision and a self-signed certificate and following this video's instructions: https://youtube.com/watch?v=4mycvUDRqg0

1. Tried binding the https type certificate to port 443 through IIS.
2. The self-signed certificate is placed into a trusted root certificate and personal
3. added network service as a user in manage private keys (mmc)
4. My URL is: https://143.###.##.##/Dynamics100
5.  both clientuser.config and web.config has the same authentication. (tried giving dnsidentity the self-signed cert name and without_)
6. PowerShell command when creating the certificate: 
Import-Module c:\GenerateSSCertificate.ps1
New-SelfSignedCertificateEx –Subject "CN=FullComputerNameHere" –IsCA $true –Exportable –StoreLocation LocalMachine -SubjectAlternativeName FullComputerNameHere

I have tried pretty much everything to make the web client work outside of the domain. Do I need to purchase a certificate to get this working? Does a self-signed certificate not suffice? Any suggestions would be helpful; hopefully my explanation was clear. Please let me know if I haven't listed something or if I wasn't clear.

Cheers