Someone asked me a question about this so thought I would share my experience
Whether connecting locally via ethernet or remotely through a router you need a secure telnet program to talk to the Mitel 200 ICP. While Mitel supplies one on the data disk with each switch there are some things it doesn't do well (or at all) so in case anyone is interested
I like puTTY as my secure telnet program as it is easy to enable logging which is very helpful for capturing the "display" of system programming in form 32 in order to have a text copy of the program. And you can direct the logging to any folder and name the file.
You can download it here - it's free https://www.putty.org/
a couple of things I have discovered with puTTY for connecting to the Mitel
Set your connection type to RAW
Set your keyboard to VT100+
Under "Terminal" set
"Local line echo" and "local line editing" to "FORCE OFF" otherwise the Esc key won't work.