Decrypt Config of size around 390 KB [ ISP Custom F/W ]

[PratikDahal7]

any ideas to know how to decrypt config (size is 390 kb, so encryption can not be that hard, ?) of isp customized firmware?
UI Looks like this :

here is the config link: config.bin
Telnet is already open but doesn't accept any i/p.

can someone help me to how decrypt the config? and send me [email protected]