The 2.4GHz band is saturated (in an apartment building you'll see tens of networks, there's no free channel even if you want to optimize. And there are not many 5GHz-capable devices or they have the 5GHz band disabled (not sure why - I found this also whilst renting/airbnb in other countries, the wifi was bad but the old "admin/admin" login worked on the router, so we could switch it to 5GHz).
And yes, the install teams are outsourced and the quality varies wildly.
Often in apartment rentals in Romania, the link to the apartment is a fast fiber one, but 1) the installers didn’t properly connect all eight pins to the ethernet cable, and/or 2) the apartment’s owner has installed a slow router. Gigabit routers have until recently been a specialist item in Romania that had to be special-ordered, and the owners of rented apartments and cafes just bought a cheap slow router at the local hypermarket. So, the internet that customers enjoyed was always slower than the physical connection could have provided.