There are a few settings on the iSeries to check but i would doubt any would be set that low.
WRKSYSVAL QINACTITV, select option 5 to display. It’s possible the setting could be at 5 minutes if this is after normal work hours and there is a scheduled job that resets this value.
The TCP keep alive parm TCPKEEPALV in CHGTCPA
and the Session keep alive timeout TIMMRKTIMO in CHGTELNA.
There used to be a setting in the .WS file in the private subfolder but i believe that was removed in v5r1?
I would verify with your network personnel if there is a similiar setting on there side.