The new router is WR1043ND V2?
What's the firmware version? Upgrade it first.
The switch is unmanaged, right?

Yes, as it states at the top of my post

firmware = latest = 3.17.38 Build 140613 Rel.55867n

Unmanaged switch, correct, is that important?

an important factor I omitted is that the modem has only 100mbit ethernet. So when the router is connected directly to the modem, the WAN port is obviously at 100mbit

Amazingly, tp-link support suggested I connect WAN to LAN-1, LAN-4 to the modem - on the face of it it looks like using the router as a switch between WAN and modem so that WAN port stays at 1 gigabit. I was reluctant to do this, as I don't really think it's ever a good idea to connect an ethernet switch to itself

But, I blindly followed the suggestion, and do you want to know what such an arrangement does???

It causes the modem to continually reboot, and malfunction even once the WAN-LAN cable is removed and even after a power cycle. The only way to resurrect it after this completely stupid suggestion was to reset.

I'm beginning to lose faith for the support person assigned to this problem, to be honest.

Lodged support ticket 3 days ago, and all support has done is ask me how I tested and asked me to create a ethernet loop (derpa).

I would've thought by now they'd either confirm the issue exists in the router, or that my router is faulty

Is TP-LINK support always this slow and technically inept?

Well, seems like a "LAYER 2" incompatibility between the router and my current modem - not sure if it's a general WR1043NDv2 - Billion 7300A incompatibility, or if one of my particular units is faulty. The router was good value for money, except now I have to buy a modem

If you use a Dynamic IP address on the WAN Connection, you may try to change the MTU from 1500 to 1492 (especially if you are on a PPPoE based connection).

Eventually, you may try using a Static IP address on the WAN Connection if your ISP provider allow you to do so.

FreePix wrote

If you use a Dynamic IP address on the WAN Connection, you may try to change the MTU from 1500 to 1492 (especially if you are on a PPPoE based connection).

Eventually, you may try using a Static IP address on the WAN Connection if your ISP provider allow you to do so.

it's a Layer 2 incompatibility