I had checked all the cables with my laptop, see above

. I decided that the modem must be goosed., and although it still has a couple of months g/tee left I didn't want the hassle of lugging it back to the shop again. I fitted a new modem myself and it is working fine. I was just puzzled as to why all the checks and tests stated that the duff modem was apparently fine when it patently was not.