@Pbaker 

So you are using the hotspot as the internet source connected to the Range Extender and then the Rotuer as an AP to the Range Extender?  Is that correct?  If so lkely the issue is going to be the ammount of devices connected to the hotspot, which is further impacted by the half duplex nature of the Range Extender.  

@Pbaker 

The likelyness of this setup having issues is high.  This is because Cellphone hotspots aren't designed to have mulitple devices connecting at a time.  On top of that a range extender will reduce recieved speed up to 50% then you have the router communicating with the majority of the client devices directly and relaying that through the ethernet connect to the range extender.  its a very unique setup and one that will not be with out problems.  I would recommend by starting to remove the extender.  On the router look for one of two features.  1.  Opperation mode for Range Extender or WDS bridging.  Both features will have the router act as the range extender.  While the speed issue will still be there you will remove 1 link in the chain and help mitgiate the latency and potental issues.  This is probably the only advice we can give in a situation like this.    I can understand the issues of living in Rural America.  My mother lives in a rural part of Roy WA where Satalite is the only internet choice and I have had to tell her to limit her device activty to one device at a time because of it.  The real fix is when the infrastructure finally makes it to you and you then finally have high speed access.