Trek Antelope 800 chain falls in low gear/under torque

I bought a trek antelope 800 that had the grips, pedals and chain replaced. It has the original Shimano system with Altus c2o derailleurs

It works in high gear , especially when front tire is in high gear, but when I have both front and rear in low gear, it seems like the chain slips off the cog on rear tire and then chain falls off front as well. It happens especially if I’m torquing hard in low gear and/or standing on the pedals.