Router kicks Wifi devices off.

Hey guys,

Just a quick one I'm sure most could answer, I'm a bit of a novice when it comes to Wifi, and my router is kicking devices off when we have more than 4, picking and choosing which devices it will allow Wifi too. We often have 3 phones, 2 laptops and 1 PC in the house which need constant connection. What do I do to make this happen?

My Router is a D-Link: DSL-2780, I'm with Talk-Talk in the UK. :)

Thanks in advance!