The reason that you were unable to maintain 70 mph up an incline was because the output of the REx generator was insufficient to maintain the battery pack's charge level. The REx engine doesn't start until the battery pack's charge level drops to 6%, and power reduction begins when the charge level drops to 2%, so there's not much of a cushion between normal and reduced power.
Many U.S. REx owners have modified settings (incorrectly called "coding") to enable Hold State of Charge (HSOC) mode and to make available the entire 2.5 gallon gasoline tank capacity which are standard in all other i3 markets outside North America. This allows the driver to manually start the REx engine whenever the battery pack's charge level is less than 75% rather than wait until the REx engine automatically starts at 6%. This increases the cushion so much that there might be no roads in the U.S. where an i3 could not maintain the speed limit.
Had HSOC mode been available, you could have started the REx engine when the charge level dropped to 75% and continued driving 70 mph until you needed to stop for more gasoline. It is unlikely that your battery pack's charge level would have dropped so low that power was reduced. With more useable gasoline, you would have been able to drive farther before filling up.
If you have a Windows laptop, purchased the appropriate Ethernet-to-OBD port cable, and can follow coding instructions, you could change these settings yourself. Otherwise, you could pay someone to change these settings. Others on this forum will undoubtedly provide the information you need to make these changes.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE