Without the front sensor cable assembly (the receiver speaker on the 13 series is actually in the phone, rather than on the display) the [[iPhone SMC Panic Assertion Failed|phone will always reboot|new_window=true]]. there is a sensor on that cable that the phone needs to hear from to prevent this.
How frequently is it rebooting? There could be another issue as well, but you'll need a new sensor cable for sure to have a fully functioning phone (although replacing this cable with one not originally from the phone means that True Tone and Face ID will be disabled without a bunch of extra work.