Many broadband (I assume wired as you don't mention fibre and its not low cost) cabinets have the DSLAM set to test and adjust /optimise the connection speed many times during the first week or two of a new provision /connection. Each new speed setting may involve a re-sync and service break
Usually it works it out and stabilises within an hour or so but if there's a line issue (intermittent connection maybe) or it gets switched off at night etc it hunts for a few days and may end up with a stupid result.
You low cost provider ought to know this as it's been the case for decades.
The network operator can access the cabinet and ask it what's going on but the re-sellers often can't or can but don't train their staff well enough for a service rep to understand the report. Your old router if still accepted will try to sync at whatever rate it had settled on - possibly lower than you are currently testing settings around on the new one
But leave it on for your 10 days and if its not resolved there's a line /cabinet fault to report and there should be a good long error log by then