@RRHYAST 

Telnet should be only open from the LOCAL network and not remotely, for that reason you shouldn't be too worried about this.

If you want to test this try a telnet connection to your external IP on Port 23 using a 4G (not your wifi) connection, my guess it will likely fail.   Telnet is likely OK for internal use only.

Newer routers will have this feature disabled / able to be disabled, but sadly this router is a bit older now and doesnt offer this functionality.