I had bounced the modem on/off a few time without waiting the usual 30 mins.
I had a situation where the DSL light was on but it would not lock sync. (According to error message from firmware)
It would not attempt to establish a PPP session and just sat there.
I had to bounce the modem 2/3 times until it locked sync & was able to get a PPP session.
After this the modem re-synced of its own accord twice over night and US G.INP was switched on.
Maybe bouncing the modem a few times convinced DLM that US G.INP might be needed ?!
(Empiric evidence can justify anything ...... if you try hard enough
)