From what you've said, you need the router to act as a wireless client (as opposed to a repeater) and potentially a bridge, which should be entirely possible but I'm not sure out of the box. I think this is known as media bridge mode in the Asus firmware, which implies it should bridge (the hotspots network would be relayed to the wired ports).
I've done this with ubiquiti unifis and OpenWRT. The client access part was fine but adding the transparency of a bridge was temperamental at best, due to specific features not being supported by the wireless chipset. Luckily I don't think you need that transparency.
Fyi a bridge usually implies two dedicated devices either end connecting two seperate physical network segments, usually seamlessly. You may not get that seemless integration (but you may). Worst case you end up with double NAT.
Somebody else should be able to chime in but basically you configure the router to connect to your hostspot and any wired client will have internet access.
Of course the suggestion of tethering via USB above is just as valid but you will be required to physically connect it. Your milage may vary with the ability to utilise the hotspot amongst additional devices simultaneously when it's connected like this.
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