You never tell them in advance you have a Mac.
And like I said before, you must be your own techie.
Wifi is cross platform and config should be a no brainer. Should only be wifi standard, SSID, password and type of encryption if any.
I fully agree with this one. Just tell them to set it up. I have to do the same thing with my Linux computers, even though the guys installing never get to touch my computers.
If I told the sales rep that I had a Linux computer they would simply tell me they could not provide services. My assumption, and it has been proven true with 100% of the tech support I have encountered working for the ISPś in Chile so far, is that they themselves can do little more than check their email.
Typically what I do with these problems is get the make and model of whatever modem they are using and then Google it to get the Manuel online in English. Typically that includes how to configure a system for Mac or Linux.
Only if they are insisting on installing some unneeded and funky software written for windows should you have any problems. Even then, the maker of the modem will likely have Mac support software on their site. I really though have never encountered a router used by any of the ISPś in Chile that a Mac would not connect to.
Last I seen, Telefonica del sur DSL connection are simply a low end stock ADSL dlink modem which fully supports Mac. At least in southern Chile. I know they have some funky Chinese version of 3G they are using in some cities to provide wireless internet to your home. We have one of their phones on this system, and it works great. Just dont move too much because the cell density is really high. They should be fine when the router is sitting in one place.
Also there are still a couple of companies in Chile that believe using a MAC address (hardware address) is some sort of security.