Doing a bit of Googling, although it is obviously recommended to go down the managed switches and VLAN route, connecting both WAN and LAN via unmanaged switches and one cable might just work. Again, something to try.
Thinking about it further, it might just work if it was only one WAN and the LAN, but I can't see how it would work with 2 WANs and the LAN though.
The other thing I was thinking of trying is.
In both buildings, there is an Asus AX82U which looking through the features, it has PPPoE Relay (I have not seen this before).
I was thinking, if the Modem -> AX82U (PPPoE Relay) -> (TP-LINK) ER605
I would hope the AX82U would also still allow some ports as a switch? I haven't seen PPPoE Relay before so I'd need to do some testing.
So, With the Asus Router with PPPoE Relay - It does allow the TP-Link to connect via PPPoE.
However, it doesn't 'send' the connection back and the Asus router stays disconnected, if I connect the Asus router via PPPoE, it kicks the TP-Link off.
Is it possible to have 2 PPPoE connections??
I am not sure what I am trying to do is even possible any more...
No, you can only have one PPPoE connection.
If the buildings are on the same mains supply consumer unit, you could use powerline adapters instead of running an extra cable, or just try bodging the cables as previously suggested.
So I think I managed to finally get the Asus to PPPoE Relay correctly and everything seems to be working perfectly now!
I did try the 'bodge method' - LAN worked but I was not getting any connection to WAN. So I assumed it needed more wires.
I also did think about Powerline adapters as building 2 and 3 are on the same consumer unit, I've tried using them before, but the buildings were built in the 1920's and there seems to be alot of noise.
But anyway, it all eventually worked out! Thanks for your help again!