My tankless is a Truma, it is only LP. I think all of the tankless systems are probably LP only.
Make sure your LP is turned on, one switch is by the fill and another switch is above the fridge with other controls - they both must be in the on position. Next, on mine there is a selector switch on the unit itself - remove outside cover and look for a switch (mine has two positions. eco & comfort) select one or the other otherwise it is in the off position. Last go to your one place monitor and select LP heater.
Since yours is an afterbuild the switches on the heater itself and at your control panel may vary.
Have you contacted the seller for instructions or did he provide any paperwork?
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