Asus XT8 - Plug 4G usb modem to node ?

[GeeM]

Hello,

I have a setup with the wonderful Asus XT8 mesh system.
My 2 routers are linked with an ethernet cable, the main unit on the 1st floor, the node on the second floor.

I would like to plug an old smartphone on the USB port to act as a USB 4G modem as a backup internet connection (failover). For some reasons, I would prefer plugging it to the node (better network and accessibility to the device). Is it possible to configurer a dual WAN with a USB modem plugged on the node, not on the main unit?

If so, do I need to set it up from the main router's interface, or do I need to connect to the node through the URL?

Thank you for your advices
GeeM