Heater turns on and heats 2-3 degress then the service heater light comes on.After about 1 minute the heater fires back up and the light goes out and it repeats this cycle until it up to set temp.

I would suggest that your landlord is responsible for heating as you rent and must ensure the heating works properly, especially when using gas.
Contanct a Rinna service agent and have it serviced at your landlords expense.

Check your stack flu sensor. Sensor can not be jumped. As soon as the heater fires hold down the pool or spa button until the stack temp appears. At 500 degrees the heater will shut off giving a code. The heater should not restart. I would also check for wire damage based on the description of your issue and the board restarting on it own. You may also have damage to the "user interface board" , hard to say with out on site trouble shooting.

Lack of maintenance.

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First check pressure over 1 bar ok then-
1. check flow - is it adequate - a little less than the cold tap ok then -
2. does the boiler initially fire up on low hot flow but alternate as u say
If answers are 1. no but 2. yes then check for a furred up secondary heat exchanger
if 1. yes but 2. no [that is the water not the heating] goes hot / cold and if the boiler goes out when the water goes to cold then it's the diverter valve, either to replace or if it is one which has a internal diaphragm-replace that

where the thermostat is located, the pump is not circulating the water

No, that does not seem correct. If all you did was replace the heat exchanger, then tried to start it up, how could everything go wrong. Unless there was another issue with the heater. Not too many people change out the exchanger, unless corrosion has caused a leak there What you CAN try to do is reset the heater. To do this, turn off your heater first-at the toggle switch inside the right hand side door, then turn off the circuit breaker to the heater, wait about 5 to 10 minutes, then reverse the order. Circuit breaker then heater, if you have the digital display, you will see flashing codes, this is the computer resetting, then you should see the normal display. Try starting the heater again. If it still will not fire, let me know if you receive an error code. Also, check to see if the temp the heater is reading is close to that of the actual water temp.

the thermo couple thing that pilot heats up is not staying in flame if knob is broke you may be able to heatup manualy to get gas valve to open some will wome wont

Take the fan power switch apart and clean the 2 point contacts. Also clean the 4 male studs that the wires plug into. That should solve the problem.