Make sure your mobile phone is using 2.4GHz for the connection. Power cycle your camera by pressing and holding the power button. Put your mobile phone right next to the THETA. Verify the camera is not connected to a different mobile phone. See other ideas for troubleshooting below.

https://topics.theta360.com/en/faq/c_00/9/

try to press and hold the wifi button for 7 seconds or longer. This is what the FAQ says for the S/SC. It may be the same for the m15.

Can I change the password for connecting a camera and a smartphone by Wi-Fi? [RICOH THETA S/SC] | RICOH THETA