Sounds like the phones are connecting to the 2.4GHz band!
Try splitting them:
You can split the WiFi bands by accessing your hub settings - to do this, you'll need to be connected to your hub and type in 192.168.0.1 into your web browser. You;ll then need to enter your hub's setting password (this can be found on the bottom of the hub).
Once you've access the dashboard, you'll need to go to advanced settings and then wireless - security. From there, you just need to change the SSIDs of both the 2.4ghz and the 5ghz signal to different names, you can just add on 2G and 5G to the end of the current network name.
Hit Save and then it should work!
Bear in mind that you'll have to reconnect all of your devices to the new SSIDs