I was using it for few months as temporary solution until Proximus installed permanent DSL connection.
The router now is used for other purpose, so I don't have that config, neither have ability to test it.
Probably they have changed something on their side.
Does it work from laptop? (so to be sure your user+pw is correct)
Do you have latest MT version? Have you tried stable and long-term? Maybe try some older version prior july? (sometimes MT updates break some things ... )
You can try to enable additional logging (debug), to see some additional stuff.
btw, I remember that some time ago (maybe last few months, maybe earlier) Proximus sent some kind of e-mail regarding Smart wifi, that they are changing something... but can't find it now, probably I have deleted it (as it's not the monthly bill