Your describing port knocking.

An interesting and (effective?) security by obscurity technique.

An example:

Host port 443 web server, but by default the port is fully firewalled unless you give the secret "knock".

So you hit port 5444 first, the router sees a request to 5444, and adds the source address to a list of allowed devices for the actual 433 port. Now, just your client ip can successfully connect to port 443 as normal.

Most security researchers balk at it as wholly dumb and ineffective. But I disagree. With port knocking I get ZERO hits to my publicly exposed SSH port 22 on my actual server. The firewall shows all the port scanners hitting my block port 22, thousands per day. But unless they hit my secret combo of ports first, they don't get to the allow port 22 rule.

More complex rule would be used in production:

Hit port 5001, then 5009, then 6120, then 4001 now you can hit your actual service port. And if you don't hit them in order, no go. To reach 5009, you must hit 5001 first, to hit 6120, you must have hit 5009, which already required to have hit 5001. Add a rule where if you hit 5008, it all closes and starts over to preclude a port scanner from triggering all your rules.

Mikrotik routers have the ability to do this with firewall rules that just trigger and add a client ip to a transient src-address list. The longer your chain the better.

Android has an app called "port knocker" that makes it easy to open the door any time automatically.

Only issue is someone watching your traffic will easily see your port knocks, but that's a local, persistent adversary, not a transient scan, so yea, I get why serious security people say meh, but the threat your preventing as an individual is the random drive by scans and brute force scripts, and it's 100% effective at preventing those.