From DD-WRT Wiki
 Regular disconnects
Some Providers disconnect their customers on a regular basis (i.e. every 24h), reconnecting is immediately possible. This becomes very annoying when connections currently in use are disconnected (ssh sessions, vpn, online gaming, ...). This can partially be solved by setting the Router to always reconnect at a given time (only tested with v24 beta).
 Possibility A, the rough way.
- go to Administration -> Keep Alive
- set a certain time for the Router to reboot everyday.
This takes a little bit more time than B since everything on the Router needs to be started up again.
 Possibility B, the elegant way.
- go to Administration -> Management
- enable cron
- set a custom command:
0 5 * * * root killall -1 pppd;
This will only restart the pppd process at 5 o'clock in the morning and the router will "dial" in again, only takes 1-3 seconds with adsl+pppoe.