I have had BT send me 5 different HH5 over the last year or so and 3 engineer visits despite telling them they were wasting their time, as they are still denying there is a problem with them.
After having read that there may be an issue with Version A (especially connected to Huawei VDSL DSLAMS which I have and compatible with G.IMP which is being enabled country wide with Huawei DSLAMS) I requested a version B. They said they could tell which version would be despatched as it was pot luck in the warehouse. You guessed it another Version A arrived, and yes it was just as unreliable as the rest dropping about once a day. So they said they would send another engineer and would try and make sure he had a Version B with him!!
He arrived, insisted on checking all the wiring (no issues as expected) and when asked about the Version B router he knew nothing about it!!!
Anyway he bought one from his van and hurrah, it was a version B. Installed, speed immediately went up and had been rock solid ever since. Coincidence? I don't think so.
So I am convinced. Suggest if you have Huawei DSLAMS in your street cabinets you should try it.