@Wongman 

If the signal light is white and both 2.4G and 5G then the extender should be connected successfully to your router. The _ext network would be the extender. Since you are only seeing two networks, it seems that your router has the 2.4G and 5G networks named the same rather than different.

With respect to your iPhone, try forgetting your main wireless network and the ext network, then connect back to the ext network. 

If you connect to the ext network but have no internet, try the following:

-Rename your wireless router networks to be different:

2.4G: mywifi

5G: mywifi-5G

Factory reset the RE300 by holding the reset pinhole for 15 seconds, then set it up again. In the end you should see four networks, two being the original router and two being the extender.