If the ramp up of SNR and you resetting the modem is in a short time frame (e.g. under a day) then this is not the DLM but standard behaviour for ADSL services).
The modem sees lots of noise, drops line, renegotiaties and connects at slower speed, the noise goes away so the margins improve, but the router will ONLY use that extra margin now if you manually restart it.
if it was the DLM you would be waiting 1 to 2 weeks or more before it decided to relent and hand back the margin.
Dusk is important as interfering AM radio signals travel further once the sun has set which can increase the noise floor and make something that was just about stable in daylight become unstable. Use of a master socket filter and a trying different modem hardware to handle the noise issue can help (i.e. some modems will suffer more errors without dropping the line, which if the noise is short duration can be better).