How do I force the modem to use GDMT?
when adsl2 + box is unchecked
usually GDMT will be selected or adsl2
Should I keep the ADSL2+ box unchecked all the time?
Yes. if problem solved.
By the way, what's the significance of these settings (GDMT & ADSL2+)?
The synchronised rate ( Max ) and actual rate differ.
for a stable connection with a tariff plan of lower speed GDMT preferred.
Even now your modem does not show speed above say 2 Mbps.
Check speed here and have a record U.K.Server. do two tests each time.
MINE Home 750 max speed 512 Kbps.
[IMG]

[/IMG]
My Broadband Speed - Broadband Speed Tester