5500w/240 V = 22.9 amps max. If you can only get up to 18.5 amps then 18.5a x 208V = 3848w.
You will need a stepup transformer to get anywhere near 5500w unless you can find an element rated 5500w @ 208v .
This might get you close https://www.heritageparts.com/Manufacturers/Hubbell-Electric-Heater/HUBBELL-ELEMENT,-5000W,-208V/p/HBLC1315-4?gclid=CjwKCAjw4sLVBRAlEiwASblR-wA_VV59-EktAPSeGs_vM2jS5x4Ph3gMgI2nWH0cpqCo3ixS97CphxoCwNoQAvD_BwE