Sending multiple wireless data at same time

I got a project where multiple nodes send 36bit data to one reciever,
I used HC12 module for this and it worked fine untill i increased nodes to more than four,
The data started mixing in the air and its recieved wrong by the reciever due to interference between nodes.
Data are sent frequently two seconds seperated,
So, some time one node sends a message while the other one still sendig!!
I guess this is the problem,

Any suggestions to fix this?
Is moving from 433+mhz to 2.4ghz could fix it?

Rather than an ad-hoc setup like you have, Get the receiver to ask each node for its data in turn.

Yes, try polling each node.

Only if the 2.4 gHz devices have an in-built CSMA/CD stack.