Better wait for someone else who knows more. You are connecting a nWireless router to a gModem/router. You haven't said what ethernet cables you have got connected or if your pc that you are setting it up on is wired or not.
My intial feelings are your pc is still connected (by cable or wireless?) to the Huawei EchoLife HG520b and not to the ZyXEL NBG4615
You need to configure the ZyXEL NBG4615 first & leave the TalkTalk connection alone.
Connect the ZyXEL NBG4615 to a spare PC/Laptop if possible with an ethernet cable ,if not you can use the same PC but unplug the Huawei EchoLife HG520b ethernet cable first.
If the ZyXEL NBG4615 comes with a setup CD then use that will make your life easier,just follow the instructions & make sure to disable the DHCP function i.e. use it only as a wireless access point .
After finishing setup, plug the ZyXEL NBG4615 to one of the LAN ports of the Huawei EchoLife HG520b
(plug the disconnected ethernet cable back in if you're using the same PC), it'll be given an IP address by the TalkTalk router.
Now you can choose which router to connect to but of course the ZyXEL NBG4615 performance will be limited by the TalkTalk one.
The proper way to do of course is getting a wireless modem/router to replace the TalkTalk router, I'm not with TalkTalk but was told this router click here works very well with TalkTalk however there are loads of other options depending on your budget.
BTW the TP-Link TD-8816 modem is a wired modem/router not wireless.