I have built-in Mikrotik vpn checked in with pwd enabled.
I have ISP router port forwarding for vpn enabled on ISP router.
I cannot connect from outside to my built-in Mikrotik vpn using l2tp connection, as that is supported on my Mac and iOS 17.

I have tested internally, (using internal ip 192.168.x.x.) I'm able to connect using l2tp connection from mac.

Any idea what needs to be done, so, from outside I can connect to built-in VPN using l2tp connection?

Suggest you use Wireguard instead. Does your home router get a reachable public IP, or the upstream router if there is one ( and can you port forward on said upstream router)