AirDrop not working in extended network environment

I have an ISP wifi router (A) that is extended using an ASUS RT-AC68 router (B) with a different SSID.

When my iPhone6 and Macbook Air are connected to B, AirDrop doesn't work because both devices can't find each other.

When both iPhone6 and Macbook Air are connected to the A, AirDrop works.

Can anyone help me understand why using B doesn't work? Is there a solution?

I do not use A because the range of its wifi is horrible, so B is my primary wifi network.

Thanks!