You're gonna need a tech out to sort it for you. Call 150 and get one booked asap.
Have a look at the back of the modem, where the white cable goes into it, just makesure you don't have an attenuator or forward path attenuator on there or something like that. If you do, this will be a circular or oblong metal device about 30-40mm long. It should also have a sticker on it telling you what value it is if you do.
If you do have one inline, then remove it (it just unscrews), that will give you a little more signal and
might get you online until a tech can come out and sort it for you.
If you don't have any attenuator inline, then there isn't much you can do short of messing around with splitters etc, and unless you know what you are doing here i'd advise against this
