I use S+ and flexible pipes to connect the cauldron to foundation pipes

try an angled pipe it should snap to the bottom of the cauldron and work

you have to place a pipe onto an internally piped foundation and place a pipe from the couldron...and flexible pipe connect those two

And if you dont use S+ ?

With various mods sometimes you have to snap the mod item to the pipe, not the pipe to the mod, try placing a pipe down first and snapping the cauldron to it instead of the other way round.