Ok I have an amount of experience with basically everything going on here so here’s what you should do:

First, find the listing and see if they have WiFi listed as an amenity. If they do great, you can complain to Airbnb as a last resort. If they don’t you can’t, which honestly probably isn’t going to change much unless they are turds.

Second, do a few speed tests around the house, especially next to the other duplex unit. On the Airbnb app, send a screenshot of them and say something to the effect of “hey we noticed the Internet is slow, are you having issues too?”

Either they never checked if the downstairs WiFi and there’s no signal, or there’s a problem with the Internet and they need to call the company. Both are pretty viable. Does your phone say 75% signal or -75db? -75db is not great, but 75% should be ok. If you get faster speeds near the other unit it’s likely their WiFi.

The other option is they have issues too. Fixed wireless can run into issues when things change like radar frequencies. They can call the company and get it fixed pretty quick. Even if they aren’t paying for the faster speeds the ping shouldn’t be anywhere near 600ms. Like, I lived with wireless internet for a long while and it’s slow or shouldn’t be that painfully slow.

Don’t just suffer through, often people don’t mention this kind of stuff and if the hosts aren’t on top of their tech they don’t know it’s an issue. There was an issue with the Wi-Fi firmware on a unit I do work for and the guests only mentioned it at the end of their month long stay. They should be willing to work with you especially if they advertise wifi but honestly probably even if they don’t. Like, just don’t be an ass about it and they’ll probably be pretty accommodating.