Telnet is a one of easiest way to connect and manage devices.

but when you try to connect to CISCO ASA with telnet from outside of ASA, you will see this.

there is limitation to use telnet. CISCO Says.

to enable telnet on outside interface, you need to change the security level of ‘outside’ interface and then you need make lower security level interface.

as you see, i configured GigabitEthernet0/3, that is non used, as ‘office’ interface with security-level 0, then i changed the security-level from 0 to 10 on ‘outside’ interface.

i recommend that just use ssh protocol for connection from outside.