Thanks for replying @gfleming. Have just resolved the issue as below.
Was able to connect to have multiple devices receive IPs from the DHCP and none were able to connect to the web-GUI despite trying in multiple browsers.
I was connected via a bridged Wifi connection due to MacBook not having an ethernet port, which may have been the issue but I have no way to confirm now.
I have now managed to get access to the device by using FortiExplorer on my iPhone and connecting to the device via my charging cable in the USB port. Once I got in on the iPhone and changed the subnet configuration everything on the network came to life and I am now able to access via the web-GUI. Literally 8 hours from my office out in the mountains, so really glad this has been solved. Hopefully saves someone else who left civilisation without the proper gear in the future :)