Ah, no idea sorry.
I do put bicarb and vinegar down my indoor drains though, but it would probably bubble up and out instead of down wouldn't it?
It looks like the drains are still running ok after their manual clean may put caustic soda down them. Also considering paying for a presure wash.
Bleach and boiling water. This was the tip given to me when I was a student and our outside drain from the kitchen got blocked. Did the trick but never needed to do it again so not use whether it works for all types of blocked drain
Dont know where you are but i used a guy Called Richard from barnes drainage and he jet washed my drains and the amount of stuff in them was unbelievable. I highly recommend and never had a problem since. Hes on facebook Barnes drainage. There isnt any chemicals that can clean drains other than jet washing. Hope you get it sorted x