In my device (j7 nxt) , when sim1 is 4g and sim2 is 3g , then i can not connect to internet via sim2 in 3g connection. when i change the data connection to sim2 it change to 4g sim1 goes to 3g. There should have a option to turn on mobile data connection via 3g, when 1 sim is on 4g and another is on 3g . Thats all.
please do needful.