Custom Login URL for Zoho Mail SSL Security Error

Custom Login URL for Zoho Mail SSL Security Error

Hi!

I have tried using the "Custom Login URL for Zoho Mail" option in the zoho control panel but ran into an SSL issue. The "mail" subdomain seams to allow an https connection but the cert provided is issued to *.zoho.com with no alias or common name for my site. (NET::ERR_CERT_COMMON_NAME_INVALID)

see for yourself: https://mail.dundeecourtcoop.com/

Is there a way to upload my own certificate or disable the use of SSL on that page without going for a premium plan? Alternatively, if the previous option doesn't exist, can I build my own login page that posts to zoho and is there documentation for this? Going premium is not an option as this is for a non-profit co-op.

Some info:
I have the domain through google domains, server space and DNS through DigitalOcean, the SSL certificate through Let's Encrypt, and I also have HSTS and mod_rewrite http to https enabled on my site.