If it has good pressure with the heater bypassed, the restriction is almost certainly in the unit or its connections. Check the inlet filter/strainer first — they often clog or get debris when a heater is moved. Also confirm it’s plumbed the right way around and that there isn’t a check valve or mixing valve installed backwards. Some hot water machines also need a minimum inlet pressure or flow to operate, so if the guest house supply is lower than the main house, the internal valve may not open fully.