This would be what I would do to rule things out?
My recommendations for you two who are suffering these problems is firstly both of you are running WiFi which will never give you has good as a connection as Ethernet.
Thats my opinion, Many other on the internet will tell you differently. :( (
wifi v ethernet speed )
Ethernet = That,s a cable directly from the router to your device. Until you connect one you,ll just be guessing what the problem is?
Now like I say connect a device other than your mag box (Under the TV) like a PlayStation/xbox/laptop since these you can check your speeds. With a cable from the router directly not by any other means, Until you do you can not rule out wifi and cabling?
This will rule out any cables or wifi has the problem. If the device still has problems then its cabling/wifi thats the problem.
If you get the speeds has tho you connected to the router by a hard wired cable then the problem is with mag box.
Can,t really help beyond what I advice. Since your telling us that ISP provider is saying your getting the speeds fine in to your home?