Check wire gauge inside water heater.
The problem could be 5500 watt lower element connected to 12 gauge wire.
5500 watt element will overheat 12 gauge wire and cause ECO reset button to trip.
To check wire size, check wire connections at top of heater. If circuit breaker is 30 amp, then wire from breaker is probably 10 gauge. If circuit breaker is 20 amp, then wire from breaker is probably smaller 12 gauge. Compare wire from breaker with water heater wires at top of heater.
Electric water heaters manufacturers today use smaller 12 gauge wire on some heaters to cut cost vrs using larger 10 gauge wire.
12 gauge is ok for short lengths and with 4500 watt elements, but water heater circuit should have 10 gauge wire and 30 amp breaker for any wattage over 3800.
The upper thermostat ECO will trip if it senses high heat on the wire, or if loose wire or loose connection causes heat.
This is one reason instruction manuals say to use same wattage element that shows on nameplate.