Do you know what happen when your line dropped and what was done to fix it?
Exactly what was faulty? Did any of your neighbours have problems at the same time?
Sounds like your line was cut and is now impacted due to a splice.
I'm also confused by your post. My line was dropping formerly on EO Line on ADSL. It would quickly reconnect within 1-2 minutes but my connection wouldn't maintain stability with a 3dB or 6dB SNR. When I contacted my ISP to manually increase the SNR to 9dB only then my connection was stable but at reduced speeds such as 12Mbps.
My line was not cut. There were no physical damages to the cable. But I believe due to the long length and noise on the line the connection would never remain stable for more than 1-4 days. Neighbours in my case didn't have problems at the same time, but they suffered from the same disconnection problems as soon as SNR dropped to 0dB connection cuts off.
My EO Line FTTC cabinet got upgraded in October 2019 and this month is exactly 1 year since I now have FTTC. My connection problem spontaneously got resolved from the FTTC switch and I now have 80/20 Mbps rock solid last 12 months. It is a 24 month contract with TalkTalk and I am no longer experiencing these problems.
My connection on ADSL was always having issues and Openreach engineers were never able to fix it. Even after my EO Line got upgraded to FTTC nearly 2 years ago my ADSL was still suffering. It was only after I had switched to FTTC following my end of the ADSL contract with Plusnet did my problem get resolved.
I know that till this day people in my area (Bishopsgate exchange) who are on ADSL will still have problems, but switching to FTTC immediately rectifies the problem. Many people here on these forums and on Youtube have had the same experiences. Generally speaking FTTC with shorter copper cable length to the cabinet reduces risk of noise and that should normally resolve the problem automatically. But OP is obviously unlucky that his length is 550 meters long, I'm guessing there is some random noise problems on his lines that is causing his drop-outs.