Hi @Carlo4890 and I really feel for you with your problem, something - thankfully - I've not experienced with our Airbnb property, although once upon a time, years ago, I did experience something similar, when tenants we had used vast amounts of hot water when having baths (as lots of hot water produces lots of condensation, especially when the same people will not open windows!)
As far as I can see, from what you have commented, this is a bit of a conundrum, inasmuch as it would appear that your current Guests are the first to stay (since you became an Airbnb Host) to have either experienced a lack of hot water, or to have commented upon it, so in a way, the current situation isn't anything to do with you*, but all to do with your Guests, and the amount of hot water they've drawn out of the system (I say it isn't anything to do with you, and yet, at the end of the day, the buck does stop/remain with you as to the who and what will be done next)
Of course, if only the one set of Guests' have complained, you might take the line that apart from gaining/losing the potential of gaining a SuperHost status, you probably won't do anything "as no-one else has ever complained". Well that is a decision you might have to gamble upon.
The alternatives (and I'm saying this not knowing what type of heating system you have to heat your water, what size of hot water tank you have, nor how long it takes for your heating system to heat the water for another full tank of the stuff) might include ensuring that the system automatically heats up the incoming water so that there's a constant flow of water that's hot.
Another possibility is to put some form of a restrictor on the water-flow (out of the tank) to ensure that the heating will be able to cope with the amount of water in the tank
The following would be more expensive methods to employ, although would be more likely to resolve the immediate problem (even though you might not be able to get the work done whilst your present Guests' are "in residence"!)
One would be to purchase and have installed an instantaneous water heater (electrical) for the bathroom (but if the amount of water is affecting other parts of the property, then another such heater may also be required for washing - dishes and/or hands).
Another possibility would be similar to the above but to remove the bath, and only have a shower cubicle, with the heating of the water for the latter being via an instantaneous (electric) shower.
There are alternatives to even those suggestions (e.g. put in a larger heating system with a larger tank; install a water flow restrictor; install a pre-payment water for Guests to pay to use); - all of which would either cost you a lot - financially - or possibly ratings-wise (although the latter could go either way!)
Of course, Of course,a cheaper means would be to include within the description of your property that as the suite runs off the same hot water system as the main property, there will be times when the amount of hot water might not be sufficient.
The cheapest of all however, might be your answer to the question as to "Have you spoken to your Guests to find out what they might be doing that is causing the problem?" as your doing that may well actually prove to be the simplest way of finding out the "why", after which - between you all - you should all be able to resolve it more easily (and hopefully in a much cheaper way)!