Diagnozing issues with connectivity

Hi,

I have been trying to diagnose my issue with internet connectivity. Multiple times in a day internet connection drops for a short time. This causing issue with internet calls etc. I’m using RB1100ax4. Are there any tips how to diagnose what can cause this issue? I know this is not from ISP side because we have a switch and another router before mikrotik with different external IP and the parallel connection works without any issues when mikrotik connections drops. Afterwards connection restores by itself and no other action needed.
Thank you