Same here, I've phoned them 9 times, had a engineer out and changed my router. Now they're saying it's a fault on my end! It's clearly not as a lot of people on here are saying the same thing. I've seen another post saying it's going to be like this until April 2017!
I am having the exact same problem! We have vivid 200 and have never had any problems, talking many years, but within the last week the signal has dropped meaning xbox is disconnecting after every few minutes. Also the wifi strength appears to have dropped, we used to get full 100%, full 4 bars, now this has dropped to low 70's and only 3 bars.
Spoken to VM support team and they have reset modem remotely, which helped for a few minutes, but problem is back again, the signal speed works for 3 or 4 minutes then drops dramatically and connection lost.
Having seen this page lots of people appear to be experiencing the same problem so there must have been a change somewhere which is affecting everyone.