Hi @david64 , thankfully from your help and the TPG's help, it was my mobile data that was interrupting it (I think) - I can connect to the wifi now 
! hurray!
But now there's another problem - on my mobile device it says that the wifi is connected without internet.
The internet works but is VERY VERY SLOW.. My laptop loads the "DNS address coult not be found" and then 10 seconds later it will load the page.
Should I switch off the modem and switch it back on?
Kind of scared to touch that modem again....@_@
TPG suggested me to do everything as normal (what you told me to do) press reset router 10 seconds > type in 192.168.1.1 > didn't work? try again using other device > then they gave me the security key number so that I can do it wirelessly > tried but still didn't work > TPG told me to make sure my mobile data is turned off (I think they said it at the beginning but I couldn't hear the sound properly) and then the 192.168.1.1 link directed me to the proper site! *didn't know by turning off my phone mobile data will fix the problem!
After that, the internet worked but the web page said DNS not responding > 5 seconds later > Internet not available > 10 seconds later > website loads successfully.
Since it kept saying "DNS not responding" before being able to fully load, I searched Youtube and followed a guide > put in a DNS number manually.
The method does seem to work however, the wifi is still switching on and off..
I was confused if it's my new laptop's problem (got the laptop yesterday) so I tried with my other devices but they took a looooooong time to load as well.
Right now, it still takes time to fully load a web page but it doesn't mention anything about DNS.
Just hoping the speed gets back to normal soon...Maybe it's the weather? I don't know..
I have NBN FTTP.
I bought a wifi range extender today and the range extender is working pretty well - other devices connected to the extender is working okay. (better than when not connected to the extender)
I don't know if this connection problem is just something that'll happen one day and go away the next day, because I experienced something similar a few months ago (internet will be terribly slow and keep disconnecting) but worked normally after a day or two.
+ I didn't change the ip address manually because I was worried I might do something wrong - but I think if I followed your first few steps with my mobile data off, it would've worked 
!
Thank you so much @david64 for helping me today, you really gave me HOPE throughout the day that I'll be able to fix this problem - if you want to know more about how I 'fixed' this then please ask me more!