I realize this might seem self explanatory, but I was under the impression that if the modem syncs, you should be able to connect, just at a lower speed. I've found however that that is not the case, the modem tries to send login and password information, but it never makes it to the ISP.
If your profile is set to VDSL2 your ADSL/ADSL2+ will not sync.
If your modem is currently in sync but it is not logging in, if Bell is your provider give 310-SURF (7873) a call and they will be able to help you by either resetting your login details or dispatching a tech if necessary. If you're with a 3rd party you will need to call there support line and they will help you troubleshoot the problem.
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A login issue can be caused by different issues, a defective port, bad credentials or a bad modem (uncommon).
It's not my DSL that this is happening to, just some people I've helped. bell accidentally switched them, causing non-connectivity, but sysnc. In both cases the line was changed, but also in both cases a temp change in modem solved the issue.