problem with router's settings

ELKALIBEY

Honorable
Jun 3, 2013
46
0
10,530
my router TD-8840T ver 3.0

everytime i change my ISP settings to PPPoA/PPPoE and click SAVE
it changes and becomes Bridge Mode instead of PPPoA and network's speed becomes slow such as ping

so how can i firm settings on PPPoA/PPPoE not Bridge mode