What suburb are you in?
Note that Optus can "tune" your ADSL line from their end. They can go agressive which has more speed but increases the likelyhood for dropouts. Or it sounds like when you complained they may have dialed the speed down to improve stablity. Perhaps contact them now and ask them to go more agressive on your line?
It sounds like we've given you the dual band Sagemcom modem. The modem has two available WiFi Networks, that's 2Ghz & 5Ghz WiFi Networks. I've found that the 5Ghz network seems to be set as the default network. Could you try selecting the 2.4Ghz Network? As a general rule the 2.4Ghz network should be used when you're further away from the modem. The 5Ghz Network does allow for faster speeds but is subject to more signal loss the further you are away from the modem.
The password should be the same. I'd try switching between the two networks, hopefully that makes a difference. If not, I'd also suggest running a speed test over Ethernet. If there's a large discrepancy between the tests run over WiFi vs the Ethernet test, that might suggest that the new modem is faulty.