Your external IP is dynamic because the ISP can rotate it. You want to reach your home's external IP to VPN in. One common way is to create a public DNS record that's dynamically updated (by a cronjob or whatever) to always contain whatever IP your ISP last handed you.

That's what I do. Just a cronjob on my TrueNAS server to query my IP and update my subdomain's A record if my IP has changed. That way when a power outage happens and my IP gets rotated, it makes no difference.