Actually this beta is for India only and you are using Bangadesh sim network so ideally you will not be able to enrol in beta program. Use your Indian sim if you have with you else wait for stable version.

Did you change the SIM card or what

I belive offering Beta in March then stable in April making no sense unless they again delay the stable for S Series.

What'll the beta do when Samsung drops the stable version in April? What bugs'll they find in a month or less? It's just a game Samsung's playing.