The RTAB-Map iOS app doesn't currently support external depth sensors. Note sure if there are many depth cameras supported on iOS. One sensor I know is the
Structure sensor, but it is kinda the same price (or even more expensive) than buying a new iPad Pro with internal LiDAR. Some
old iPad Pro with LiDAR seem to be under 600$, which is hard to beat to avoid struggling with an external sensor (that could have synchronization and calibration issues).