# Permit any forward traffic that has a destination NAT state.
/ip firewall filter add chain=forward connection-nat-state=dstnat action=accept
# Destination NAT any traffic on port tcp 23 to the host 192.168.1.30
/ip firewall nat add chain=dstnat protocol=tcp port=23 action=dst-nat to-addresses=192.168.1.30
# Disable telnet on the gateway router to prevent port conflicts.
/ip service disable numbers=[find where name=telnet]
You should take a few things into consideration though, if you take this approach you will be connecting over the internet via Telnet. Telnet has no encryption mechanism so all data will be plain text. This means that anyone in the middle of that connection can see your login credentials and compromise the device.
I would strongly recommend using a VPN technology with IPsec such as L2TP/IPsec or even IPsec XAuth to securely connect to the gateway device, then you can Telnet directly to the device using the local address 192.168.1.30 since you would be tunneled inside of the local network. If you already have Winbox access to the gateway router, you can also go to Tools > Telnet click the Telnet radio button and insert 192.168.1.30 into the address field. This will allow you to Telnet into the device from the MikroTik on site.