Router is sort of limiting how many wireless devices can join simultaneously (NO DHCP issue)?

I have TP Link router which is working perfect. No problem at all but recently I have a bunch of more devices coming in and trying to connect to the same wifi spot and the problem comes in. It seems like router is consistently trying to limit the number of devices it will serve simultaneously. The reason why I am saying this is I tried connecting my ipad to the router and iphone at the same time. Ipad goes in but iphone gets stuck. When I turn off wifi on my ipad then the iphone gets online. I know this is a bit confusing and obviously it is not DHCP problem as I have pool of IP allocated for more than 80 machines and I am barely using less than 20 (including gadget and such).

Any pointer and input will be very much appreciated. Thanks.

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