Welcome to OBi-world!
You're making the "noob error" that everyone makes, because it isn't obvious: use the OBiTALK gateway to make changes, in "Expert" mode. Don't use the local web page (192.x.x.x or 10.x.x.x). You must un-check BOTH boxes to the far right of the row to enable editing. Click "Submit" at the bottom of the page after editing. Wait a few minutes for the portal to send the new configuration to your device.
Aside from that, the other confusion that you've fallen into, is that the TX and RX settings are counter-intuitive. "RX" is the audio signal that the OBi is "hearing" from your telephone's microphone; hence, it is the value to change when people can't hear you loud enough, or too loud. "TX" is the audio signal that the OBi is sending to your telephone's speaker.
Everyone's telephone hardware and human hearing are different, so your settings will vary from default. I have a modest hearing loss, and I find that setting TX and RX to -3 works for me and for the people on the other end of the call.