Modem/routers to do ADSL2+ with 4 cabled network ports and 802.11g or n Wifi are practically throwaway items these days.
I've got 2 spare myself, as such the old DLink one I had that was playing up, I just threw out.
You can still pay stupid money at some places though, for a run of the mill modem with wifi and 4 cabled network ports the price range is about $35 to $90.
Most modems should work with all ISPs but for whatever reason the Belkin one I had has a firmware update that was specficially done for TPG.
If you buy straight up retail then there should be no problem with it being locked to a provider, not sure that the ISPs themselves bother doing it any more with ADSL modems given they're so cheap for the basic ones.
As for what brand. Personal experience says Belkin and DLink is cheap-assed junk. Netgear is fairly decent, and plenty of people like TPLink.
Given that some places are imminent re NBN connection I wouldn't throw too much money at an ADSL modem. In fact I wouldn't even bother buying a new one, paid about 10 bucks for the current and a previous one I had (used) and the one in between is a Netgear that was rescued from being thrown out.
Chances are your local Gumtree ads is full of half decent used ones. Barely worth buying used from anywhere not local, postage would cost more than the item.
Edited by Rybags, 28 May 2016 - 07:32 PM.