My wifi works perfectly with everything in my home except my iPhone 5s. It connects and stays connect but the connect is so laggy when it comes to facetime it is almost unusable. I've tried resetting the modem and the network settings and all that

My wifi works perfectly with everything in my home except my iPhone 5s. It connects and stays connect but the connect is so laggy when it comes to facetime it is almost unusable. I've tried resetting the modem and the network settings and all the other things everyone recommends but still bad speed. I have a speed test app installed and I get Ping 45ms. Download speed 0.71Mbps and Upload 0.61 Mbps. The things is my internet is supposed to give me 125mbps. we just switched to it and the modem is essentially brand new.