What is the best internet modem for me?

So, first off you should know, that I'm a novice when it comes to internet and home networking technologies.

I'm an AT&T customer and I have what I believe to be a 2Wire 2701HG-B Gateway DSL router modem. We had some other router before that which died on us and we replaced it with this one. At that point, I believe we only had 1 desktop computer.

Currently, we have a desktop, a laptop, an Ipad, and at least 4 Iphones connected to this one router at any given time. I've noticed that our Wi-Fi has been slowing down and I assume it is because of the increased workload. I'm also assuming that, the router is only meant to handle 2 devices at most.

I'm thinking of getting a second Ipad and a PS4, that I'd like to connect to our Wi-Fi network. I assume I need to buy another router to accommodate these other devices.

Am I correct in assuming this? If so, what should I look for in a router. I believe the current range on the router is good enough.

Thank you all in advance for your time.

Added (1). I don't want to call AT&T because I don't want to be limited to only the modems they have available.

Believe it or not its the internet speed you purchased. You can connect up to 10 devices "but" it depends how fast your speed is. My computer is blazing fast… Until I click a video on youtube. And it made me feel like my desktop i built was useless. I purchased a higher speed… Infact i'm at 75mbps and huge difference. **** loads ands closes… Things download or don't… But i'm not waiting to see. Hope this helps.

ADSL2+ or High speed cable modem.