If your PPP is broken or you want to reconnect it do not try to use re-test in the diagnostics page as it doesn't do anything (its as if all it does is re-report the existing results).
The only way's to force a PPP re-negotiation attempt I know are power-cycle, re-boot or line disconnect/reconnect.
Also It seems as if the initial login attempt fails after DSL is up it doesn't re-try.
Finally if you are suffering severe uplink errors don't be tempted to restart your modem (as I did) as whilst IP will still transport data slowly due to re-transmissions your PPP login attempt may fail........and of course then you have no link at all!!
One of the better modems I have had, at least it has diagnostic facilities and is supported by DSLSTATS etc, these seem to be fast dissapearing from more "up-to-date" modems sporting ludicrous wi-fi speed claims etc.
W8960n on Lonnggggg line