Reverse proxy might be necessary as you are likely behind Carrier Grade NAT. Take a look at my wireguard guide and get a cheap basic 1 GB linode
Then just use a proxy on it that points through your wireguard tunnel to your server. Have DNS for your mail server point to the linode
Bare in mind you must tell linode that you will secure your own email server and not be a propagator of spam before they open email ports on their network themselves