@Iwearsocks
Please ensure that whether you can get internet from the new modem directly; if so, the internet connection type would be dynamic IP; while considering the TL-WA801ND is an access point, to share the internet with multiple devices, a regular router is needed;
If there is no internet, that means you need setup the router with username and password given by ISP to make it work, which TL-WA801ND does not capable of, you still need a regular router to make the whole network work. Like Archer C7, Archer A7, Archer AX1500 and so on.
May it help.