These little instantaneous gas hot water heaters work only if:
1. they are less than about 10m from your hot water outlet,
2. you have a standard outlet, cannot use a water saver shower head,
3. the pipes are not too restrictive, no narrow plastic flexible shower pipes.
4. They are properly serviced about once a year.

Otherwise they give poor and varying heat and the heater can turn off at random. That info cost me about 5 visits from different plumbers before I replaced the one I had.
As ‘Kato’ says you can try the temp control lever, on the cheaper types moving it to colder gives a higher but cooler flow. You could be lucky and this may work.
The bad news, blowing up a gas heater can be bad for your tenancy agreement, and the position of your head on your shoulders.
The good news – ‘summer is acommin in’.