Why my OPPO N3 Dual Sim detect either 1 sim only?

OPPO N3

Actually it work fine before until recently when I change to a new Sim Card (I change to a new mobile number - 1 sim only).

After open the slot and activated it. Its work fine. Both line was ok.

Then suddenly when I change network. Sim 1(A) to 4G, auto Sim 2(B) to 2G. My sim 2 doesn't have connection at all. Then I change sim 2 to 4G, its have connection and sim 1 working fine with 2G.

Then I change Sim 1(A) to Sim 2 (B) so now Sim 1 (B) and Sim 2 (A).

Now in this, Sim 2 (A) doesn't work at all...either 4G or 2G but Sim 1 (B) works fine both of it.

If I install Dual Sim apps. Its said it can't support because this phone doesn't support dual sim.

Please help me!

I've format my phone - still same problem.

I've flash my phone - still same problem.

What could have wrong?

Now I know why another slot is not working.

Because there no IMEI on that slot.

I don't want to root my phone but how to fix the IMEI without rooting?

I know both IMEI.

If I dial *#06#, its still show both IMEI but if I check Dual Sim App IMEI. Only 1 IMEI retrived.

Anyone know how to fix this?