What will be with MySQL cluster if network problems between nodes? I have 2 datacenters, each DC has MGT, SQL and Data nodes. 

How will MySQL NDB Cluster work if there are network problems between 2 DC? 

In this case 2 DC will work well, but they don't know anything about other node. 

Who ll be the main and who ll be the reserve, how will they decide it? 

Is there a quorum server for MySQL Cluster?
It will be decided by an arbitrator node which is normally a MGM node.
It would be advisable to put this arbitrator node in a 3rd DC.

In the case of 2 DCs the DC where the arbitrator resides will survive
if the network between the DCs is broken. With a 3rd DC with the arbitrator
node, the one to first ask for arbitration will fail.