I'm assuming it's the HT2000w modem. Which wifi signal are you trying to connect to, the 2.4GHz or 5GHz one?

On your laptop browser, go to http://192.168.42.1 and login to the wifi settings using the default 'admin' password (you can change it later under Adminstration -> Password Settings).

How are the wifi channels set up at the modem from the main page? Recommend the following:
1. Security: WPA-Personal

2. Mode: WPA2

Also, from Advanced Setup -> Wireless:
1. 2.4 GHZ should be set to Wireless Mode 11b/g/n and Bandwidth 20/40.

2. 5GHz should be set to Wireless Mode 11a/n/ac and Bandwidth 20/40/80.

3. You can also set the security and mode (from above) for each band independently under -> 2.4GHz Primary Band and -> 5GHz Primary Band.

On your iPhone/iPad, also recommend the following:
1. Auto-Join
2. Configure IP: Automatic

3. Subnet Mask: 255.255.255.0
4. Router: 192.168.42.1
5. Configure DNS: Automatic
6. Configure Proxy: Off

There are some speed advantages to using 5GHz over 2.4GHz, but 2.4GHz tends to be more reliable at a further distance.

Let me know if you have any questions.