@ChillMan I have been doing this for years now. So far best has been a DD-WRT flashed  linksys router WRT350N  I have done this two ways fIrst from a hotspot to ethernet LAN Linksys  and then  lynksys router as WAN To lan etehrnet and hotsopt.  The second was with a Linksys DD-WRT enabeled Linksys connected directly with ethernet input from a Verizon MYFI Also have used a ASUS RT-N13U with factory software. As these routers are relitavely slow to current data transfer  I am now looking to upgrade to faster routers. There are commercialy available DD-WRT enabled routers and lots of documentation online. I have also used  D-Link Routers but they were very lite duty and in short order did a system slowdown to where a turtle could deliver data faster.  You apparently need the simplest system. Buy a ethernet enabled wireless router. As you can tell I prefer DD-WRT Flashable or flashed.  Most mobile hotspots are slow so you don't need too fast a wireless router. There are also newer wireless routers that will give you wireless home network expansion and lan as output capabilities. Then connect from the LAN to the WAN on your existing router.  You should be able to get a DD-WRT flashed Router for under $25 on Ebay

@ChillMan sorry for My earlier diatribe in short to your question go to 

https://www.smartdomotik.com/2014/09/13/how-to-use-dd-wrt-router-as-wireless-adapter-client-mode/

this should set you on the rite track