I have noticed that his attenuation is higher than mine and his SNR is lower, shouldn't my speed be better than his considering my Attenuation is Lower
All other things being equal, yes.
However, with VDSL2, all other things are
not equal. In particular, the state of your line compared to the state of his line, and any differing levels of crosstalk.
Your line, for example, could have a slightly corroded joint. And/or it could be being hit by more crosstalk from other subscribers. These can be significant inequalities.
also I wonder why my SNR is 3x as high as my next door neighbour.
There are probably two main reasons why this figure differs (although you probably mean SNRM, not SNR; the M is significant)
Remember that a year ago, everyone would have a target SNRM of 6dB. You would have expected everyone to have this SNRM, unless there is a good reason.
a) Your line looks to be banded, with an artificial cap set to the downstream speed of 22.4Mbps.
This is somewhat less than the attainable speed; when actual speed is lower than attainable, it shows up as a higher SNRM than the target 6dB.
In your case, being 4.4Mbps below the attainable translates to 3.8dB SNRM above the 6dB target.
b) Your neighbour's line is currently running about 1.5Mbps above the attainable speed.
The reverse argument of (a) applies here: when actual speed is higher than attainable, it shows up as a lower SNRM than the target.
In their case, being 1.5Mbps above the attainable translates to 2.1dB SNRM below the 6dB target.
Comparing those two outcomes suggests a 3rd factor might be in play here... "XdB"
c) Up until earlier this year, BT set a 6dB SNRM target for every line. Now, however, they employ a system colloquially known as "XdB", where they adjust the 6dB target if line conditions are good. DLM monitors the line, and decides whether it could cope with (and gain from) a 5dB target instead. Or 4dB, or 3dB.
Your (capped) line will not be eligible for this system, so your target will be 6dB still.
Your neighbour's line
might have been set to a lower target SNRM, and my conclusions in (b) would need to be adjusted:
c1) If it were set a target of 5dB, then it would be running 1.5Mbps above attainable speed, while being 1.1dB below the 5dB target.
c2) If it were set a target of 4dB, then it would be running 1.5Mbps above attainable speed, while being 0.1dB below the 4dB target.
On balance, I'd say that (c1) was more likely, then (b).
Is there anything I should point out the Openreach engineer, to help them investigate the problem.
You can mention the difference in speed, but there is little else that can help.
Presumably Plusnet's support staff ran some line tests that showed a fault. Is that why an engineer appointment was booked?