What are potential causes of internet malfunction? I'm using Asus Zenfone Max Pro M2.
Running on android pie.
Phone is updated to latest firmware.
Few days ago I've changed sim card but problem still persist.
I can use internet with another sim card even with same network.
Sim tool kit is disappeared with sim I face problem, for other sim card it is appeared.
So what should I do to get rid of this?
Here is may you get cause of problem

I was facing same problem in my 3G phone so there also sim toolkit was disappeared.
And because of removing and insert sim in slot scratching sim card chip.
I insert this sim card in my new phone.
And I've started facing same problem.
So I had changed sim card but still facing same problem.
I had tried in both sim slot.  Tried other sim that was working fine.
Just notable difference is about sim toolkit.
For my sim card it is disappeared for other sim which is used in other phone is appearing.

Do I need to change my sim card as well as phone sim slot?
Possible causes of SIM Card and Mobile Data related internet problems,    


Incorrect Mobile Service Provider.
Dirty or Scratched SIM Terminals.
Misaligned SIM in SIM Slot, usually caused by cutting your own SIM Card into a Micro SIM.
Broken SIM Card.
Broken Device Aerial.
Incompatible SIM Slot ( Micro, Nano ).
Incompatible SIM Frequencies, usually caused by having an old 2G Sim card with newer 4G device's.
Mobile Phone Number Inspection, A possible DOS or Denial Of Service may be happening for a specific phone number only.


Without further information I am unable to answer you with regards to your specific problem, 

Try additional information, 
What was happening prior to the issue ? ..
Did you get a new device because of internet issues ( maybe it wasn't the device but the actual SIM Card is damaged ) 

Did it ever work at all, Can you Read SIM Contacts ( Therefore Connect to the SIM )