Hi
Temperature does make a difference to resistance but it isn't very much really and I doubt attenuation is changing due to heat affecting the copper wire. (see
https://www.cirris.com/learning-center/general-testing/special-topics/177-temperature-coefficient-of-copper)
It should be noted that attenuation reported by the router/modem is not normally a fixed measurement, but an average attenuation based on all the frequencies you receive which varies due to noise, and higher frequencies are attenuated more than lower ones, so depending on the range of frequencies the modem decides to use, the reported attenuation will vary.
The true attenuation (as used by BT) is measured at a fixed frequency of 300KHz, and in my experience on two lines I've closely monitored this has never changed.
That's not to say heat doesn't play a part. If this is VDSL then the cabinet may be baking in the sun, and hot electronics struggle and can create more noise, plus your own equipment will be running hotter as well and be a bit more noisy.
Also depending on what other customers come online, go offline, or even go away on holiday turning their modems off, means that the connection profile is always changing.
The overall trend though for ADSL and VDSL is as time goes by, sync speeds tend to worsen due to cross-talk and more people moving to the same technology, and it never gets any better. If you are lucky and have a short line length then you have enough margin to ride the storm so sync speed isn't hit but you will still notice the downward trend in SNR, but for most of us, it only gets worse*
Regards
Phil
* Sometimes it gets better, for example a change in the technology such as G.INP or a fault being fixed, but generally the trend is a worsening picture by time.