keep alive is always 300 sec.
For Tera Term v.4.78
Setup for TCP/IP keep-akíve = 1 and teraterm.ini
; Keep-Alive packet sending interval on telnet connection (per second, 0=disabled)
=> keep alive is 300 sec.
It seems that the keep-alive value is not updated.
I confirmed the value of TERATERM.INI appear in TCP/IP dialog.
How do you confirm the value is not updated by TERATERM.INI?