A rheostat modulates the power reaching the appliance, in this case your UTH. If the UTH is connected directly to the power supply now and is not providing enough heat, you may want to consider a larger model. A rheostat can't provide more power than it is supplied with initially.
It sounds like you need a larger UTH with the rheo connected inline. This will allow you to raise temps to acceptable levels. Use caution when increasing though, always allow for the temp to stabilize after upping the rheostat in small increments. Too large an increase or too many in a short period of time will create the Namibian situation instead of the Amazon.
Revenge is a dish best served cold...
With a side plate of steaming entrails,
And a nice Bordeaux!