I've done quite a bit of Googling, and it seems that people just magically get it to start working or they drop off the face of the earth. Using the settings I've been told to use, it still doesn't seem to work.
I'm pretty sure it's the latest firmware version (though I have had to reset the router a few times to make sure I wasn't doing it wrong).

Here are the PPPoE settings (I've filled the username and password section with random stuff, as I was told that this didn't actually need to have anything important in it). Very specifically, I've left the 'Service name' section blank (as apparently that was a problem that some people have had).

I've also set up the VLAN as such (I've tried priority 0 & 1, made no difference).

I've tried different cables, I've made sure that it's plugged into the WAN port, the ONT is lit up perfectly fine. I know that the ONT is working, because the router that VF gave me works (obviously a little slower over WiFi instead of ethernet). I want to use the Linksys because I can wall mount it, and because I already have it and I know it works well. I've also disabled the firewall on the Linksys, just to make sure that it wasn't getting in the way.
I feel like I've gone through everything I can, and it's not working. I'm about 3 steps away from an animal sacrifice to the networking gods to hope that it works.
Also, this is what the router that VF provided looks like on its network settings page:

This is what it looks like on its network wizard page (it automatically selects that IPoE):

Does anyone have any advice? It'd be much appreciated.
Cheers