Use Cellular Data while Maintaining Connection to Offline Wifi Hotspot

For a project I'm doing, I want to maintain a connection to an offline wifi hotspot. I want to do this so I can interact with the device wirelessly. I want to also, at the same time, be able to use my phone to browse the internet. So Device A : 100% offline. Wifi Hotspot Emitting. Device B : Phone. Connected to Wifi Hotspot. Use Cellular Data for internet. Is this possible? I have the newest iPhone with the newest iOS if it helps. I have Wifi Assist turned on in Cellular Settings, but it seems that just puts you on Cellular when the Wifi internet is poor (thus killing my connection to Device A).