Network speed is dependent on each hop in the network. First your wireless link (phone to access point) needs to be tested. Then there's the link from the access point to router (if you have separate devices for that). Next there's the link from the router to the modem, then modem to ISP local network, then ISP local network to the rest of the Internet. That could actually be different links depending on the route as well. Any one of those could be slow or having issues, and are dependent on other people who might also be using the link at the time. Your ISP might be only testing the link through the modem to their local network, which is technically correct, however if they have slow Internet uplinks it's a bit of false advertising, which pretty much all ISPs are doing now.