Can't access 4G Data Connecton.

I have 2 sim in my Samsung Galaxy A13. One is Robi and the other is Skitto. When I turn my data connection on on Robi I get the 4G network but when turning data connection on on Skitto, It always show 2G network. Even I changed my sim' slot moving one sim from Sim 1 to Sim 2 and Sim 2 to Sim 1. Why this is happening? 
There is no problem regarding network in my area and I have changed places several for last 10-15 days, moving to Sylhet town and stayed there for 6-7 days.......
What shall I do?