AFAIK, the 'max' reported by Netgears isn't documented anywhere, but it seems to be just an extrapolation of the actual speed and the actual SNRM, to indicate what could be achieved if the router reconnected with nothing (high target margins, or caps), to impede it.
I may be wrong, but it seems to me like you have two problems, which have to be tackled in sequence...
1): You have some kind of interference on your line, that renders it incapable of achieving the rates it ought to be capable of.
2): Because of (1), you have been placed on a fixed rate of 1088 (download). After resolving (1), this may resolve itself, otherwise, complain to Sky, but until (1) is fixed there's no point in complaining.
One way to tackle (1) is with Routerstats. If RS doesn't shed any light (and sometimes it doesn't) there are other things that can be tried,, but it's best to start with RS. regardless of whether you had any actual problems whilst sampling, RS may also indicate whether you had any 'near-misses, which can yield valuable clues.
That said, have you actually optimised your wiring yet? I also have attn of 55dB, and used to get about 1 to 1.5 mbps. After disconnecting the ring wire, and a few other minor mods, that increased to 4 to 5 mbps.
It's interesting, mind, that the entire street is suffering similar problems. I lived in a new build estate myself some years ago (pre-adsl, late 90's), where the service wiring installed by the builders was so bad that bedside clocks ran fast and washing machines stopped working sometimes, caused by arcing in the street-light wiring. Any chance it could be related to the on-time of street-lights? There are many things builders can screw up until they're adopted by the Councils.
- 7LM