The obstacle is not that Telnet is not enabled on your Mac; it should already be enabled. The obstacle is that the router's port 23 is closed, as it should be as well.
For example, this is what I get when I telnet to a device on my LAN:
Johns-MacBook:etc john$ telnet 10.0.1.102
Trying 10.0.1.102...
Connected to 10.0.1.102.
Escape character is '^]'.
*********************************************
* Welcome to D-Link Wireless Print Server *
* Telnet Console *
*********************************************
Server Name : PS-A45B2C
Server Model : DP-G301
F/W Version : 1.30
MAC Address : 00 15 E9 A4 5B 2C
Uptime : 8 days, 22:35:10
Please Enter Password: *********
[Main Menu]
1 - Server Configuration
2 - Port Configuration
3 - TCP/IP Configuration
4 - AppleTalk Configuration
5 - Display Information
6 - Tools
7 - Wireless Configuration
8 - Save Configuration
0 - Quit
Enter Selection:
Another example:
Johns-MacBook:etc john$ telnet 10.0.1.103
Trying 10.0.1.103...
Connected to 10.0.1.103.
Escape character is '^]'.
Lantronix EPS2-100 Version V3.7/1(031017)
Type HELP at the 'Local_6> ' prompt for assistance.
Username> john
Password> *********
Local_6>
If Telnet were enabled on your router you would get something resembling those examples. Instead, this is what happens:
Johns-MacBook:etc john$
Johns-MacBook:etc john$ telnet 24.186.35.155
Trying 24.186.35.155...
telnet: connect to address 24.186.35.155: Connection refused
telnet: Unable to connect to remote host
Johns-MacBook:etc john$
Same as you. As I said I doubt telnet is enabled anyway, but using ssh results in the same:
Johns-MacBook:etc john$
Johns-MacBook:etc john$ ssh 24.186.35.155
ssh: connect to host 24.186.35.155 port 22: Connection refused
Johns-MacBook:etc john$
That happens because your ISP does not want me access to their equipment, and neither would you. If it were that easy I would be one step closer to your Mac and everything else on your LAN. That's why I believe you need to investigate port forwarding on your router (not your ISP's) to make a Minecraft server work.
Wish I could be more help, and I have enough points already 🙂 What I don't know about Minecraft could fill a book though, and that's why I think you need to investigate this on a Minecraft forum.