This is an ancient, obsolete, and unsupported, and highly vulnerable version of OpenWrt. It is highly recommended that you immediately upgrade to a newer version -- at least 19.07 but preferably 21.02. This is assuming you have revision 1.1 or higher.
Sure. This could be done -- you'll basically setup the router to operate as a wifi client (connecting to your phone) as a wireless WAN connection, and then it can broadcast a normal SSID and offer wired+wireless access for your LAN. Beyond a quick test, though, I'm not sure how practical this will be, and your router is old and may not have great performance in this mode.
Finally I have OpenWrt 21.02.1 on all my routers and all networks up and running, all guest networks and all rules and port forwards sorted. I remember why I left if on V15 for so long man that was hard!
Anyway, now I have the latest version I would like to go back to my original objective.
So what I have to do now is as follows,
Setup the router as a WIFI client.
The Router then connects to the hotspot from my phone.
So how to set the router as a WIFI client?
Do I create a new interface (under network) to do that please?