This is my number one pet peeve and I struggle with communicating check in procedure to guests since day one.
I totally feel your frustration.
I have concluded it's up to me to make explicitly clear and get in writing that the guest has read the most important things.
This means having a personal interaction and sometimes voice phone call with every guest to make sure they understand and get confirmation in writing on the platform. I have resorted to phone calls, welcome packs with photos and images, pictograms, and even youtube videos.
With experience, I now have a 6th sense and can tell which guests are going to be a problem. People with no 4G coverage, no European cell phone package and who are not phone-tech savvy are big red flags.
I see you are in Portugal, I'm in Spain. Travellers from other continents, North America, Middle East and Asia are at a high risk for this kind of misunderstanding because of lack of phone coverage, language barrier, and different hospitality expectations and customs. If I get a guest from outside Europe I always make sure to make an extra effort and personal phone contact to ensure expectations are managed and they know what to do.
Some things I have resorted to are
(1) verifying guest phone number on booking and making sure it is the phone they will be travelling with (some people put land lines or use another phone when abroad or booked on their husbands accound or whatever).
(2) get their travel itinerary, flight NUMBERS not just "i'm arriving at 3pm" and ETA.
(3) put arrival days in red on your calendar and stay near your phone around ETA.
(4) Reaching out to them CLOSE to their trip, make sure they can get on the App and use it to communicate with you, send them a reminder about check in instructions - a day or two before - they are usually motivated to communicate with you then.
(5) Switching to a property manager that is used to handling Airbnb clients.
Basically, if this happens, you have to search for ways that it won't happen again. You may have to change .your check in procedures, make more explicit instructions, change your property manager to one with easier system.