My laptop connects to Wi-Fi and works, while other devices connect but do not?

I've googled this like mad and found no solution! I'm not the best with technology but I've always been able to set up home networks in the past without trouble. I'm apologizing in advanced for my basic terminology: I have a modem from comcast, and attempted to connect it to a router (TP-LINK TL-WR720N) using my laptop. Seemed to have worked alright, I unplug my laptop and am able to surf wirelessly no problem. Then I try connecting with my other devices (iphone, roku, and chromecast). All devices see the network, and 'connect' to the network. But my iphone can't pull up things like netflix and pandora, saying there's no internet connection, and will not work until I shut the Wi-Fi off. The chromecast is listed as offline when I try to cast to it from my laptop. The roku just tells me there's an error and won't play anything. I'm so frustrated I have tried everything I can think of. I've restarted everything many times, I've reset the router, I've re-set up the network, I've tried making the network secure and unsecure. My router software is showing these devices as connected, sending and receiving packets just fine. I've made sure MAC filtering is disabled, firewall is disabled, DHCP is enabled and UPnP is enabled. For the love of god I do not understand!

Added (1). I do not believe the problem is with my ISP because I'm getting internet via Wi-Fi on my laptop just fine.

It sounds like a master reset is what's needed. Open up your router's IP and there should be a setting to reset everything. If you don't want to do this, change your router's security to WEP (it may be on WEP2 or something). Not all devices support new levels of encryption like WEP2, so WEP maybe the best option ;-)
Hope this helps.

You said you have a modem from Comcast. Is it one of their router/modem combos? If it is, it's possible the issue is that you're trying to use it with a home router and are getting conflicts (two routers can't talk to each other very easily without specific configuration which won't come stock). If your modem is a modem/router combo AND you're trying to use your own home router, you need to call Comcast and ask them to place your modem/router into bridge mode (you likely won't be able to do it yourself).

After that, it'll just be passing all network traffic to your router and things will work better across your home network, even if it isn't the cause of your particular problem.

edit: My suggestion, if applicable depending on your hardware, doesn't involve the ISP beyond having them remote into your modem/router combo because Comcast doesn't let you have any control over it. If this is the problem, it would let some traffic through and prevent other from getting anywhere because the two routers would be trying to route things to the wrong places since both will be pushing local IP addresses to each other.