Hello,
I am afraid there is no option to set the power at 6dBm, as you have correctly indicated, power can only be set closest to at 4dBm or 8dBm.
I recommend consulting at Teltonika Telematics crowd support. These Eye sensors and the app to control them are made by another Teltonika branch, Teltonika Telematics, and it is out of the scope of assistance we could provide.
Link to their crowd forum:
https://community.teltonika-gps.com/
Sincerely, Justas.