What else is to be done if players falls through the floor?

Hey there! If we have a player and when he walks around, he sometimes 'collides' with objects...but especially, the small mountains/butes....like if I move around a 'push' on purpose on the floor...I fall through it.

This is clearly a collider problem....but I don't understand, there is a mesh collider on the ground...and it still does that...I tried making a 'box' collider. instead..., it seems that works better; but even there the player 'may go through a floor/object'...with 'pushing on it' continuously..thus, object/player intersecting problem.

This is realted to the physical engine...I am starting to think these are natural limitations of hit collision (of this physixs engine), I read that the Havoc engine could mitigate that..but how do you 'proof' the floor to never make the player go through/fall through it by gravity...I noticed when I crouch in the game...and 'push' repeatedly, the engine can have difficultly and then I fall through the floor...like as 'Update' and 'Fixed Update' for the physic engine (in Project settings > Physics)...can't keep up and so CPU may 'skip' a frame calculation and so the object 'passes through/intersects' and can literally 'Fall' trhoguh the floor...

I tried several different settings in the Physics project settings....made no difference; I activated 'collision callbacks', dynamic collision, and so forth tried them all..all of them makes player fall through floor. It is as if the Mesh Collider on the ground/floor is 'transparent/empty/non-blocking surface' by times...and only works if you 'don't push' too much on the floor...like crouching and moving like crazy...

If I walk slowly and just 'forward' never pushing down on the floor...it works...like just that...as soon as I jump, do crouching, crazy moving..then I fall through...the physics engine can't keep up with the movement collisions if 'it,s too fast/too much', so that is when 'objects/player' go through each other/intersect and 'falll through floor' in a endless fall (by gravity)...

I tried with more physics iterations/calculations CPU....which slows down frame rate but makes a more precise physics hit collision calculations...made no difference.

How can we make the floor 100% 'waterproof/fallproof/untransparent', so that the player Never Falls Through the flloor...

I read that there is not much to be done about this problem; like there are some scripts that helped, I added several 'box colliders' under the ground/floor..and that works...but I still can 'go through' mountains/butes/duners....if I push in them...if I go through them, only the invisile 'under the floor' colliders 'hold me' from falling by gravity..but I'm still 'out of the floor'...walking in the air on an 'invisible' box collider

I am thinking only switching with Havoc physics engine might solve this...because the PhysiXs Unity physics engine is not capable (it seems?) of making the player/objects Not intersect and fall through each other during the intercollisions of their respective surfaces....

Any help is greatly appreciated and happy holidays/new year 2021! Thank you very much.

PS: ''continuous detection modes'' for collisions...tried them all. not working. ''change Update to FixedUpdate''...tried, no difference.