My bass guitar isn’t triggering all notes

Hi, I am new at this type of technology can anyone can help me I just have bought the midi guitar 2 and I have hooked it up with irig hd2 with my iPhone XR using a bass guitar. It works ok but when I go to lower notes it doesn’t trigger the sound I can only hear the bass but not the sound that I have chosen. What am I doing wrong? Is there any setting that I should do on the app or something to do with my bass guitar

MIDI Guitar only converts down to guitar’s drop D.
For bass low notes, you can only use the monophonic MIDI Bass, it is included for free with MIDI Guitar full desktop version. Trial of MIDI Bass is on