Any standard mini-PCIe (but not the now current M.2/NGFF) card will work.  Intel produced many of these, but they're all out of production;  if you're looking for the latest WiFi 6 support, you can still find it -- but not with Intel cards. Example: [URL] What router model are you using that is not connecting?  
Hi,  I have an 8 year old Inspiron 15R 5537 that will not find or connect to a new router on either 2.4 or 5. All other devices including older PCs have connected to the router successfully. I have installed a new Intel N7260MWM card and still this particular router is not found. It sounds like an issue with the router, but still the question remains, what is the latest card that this system will support? Many thanks
Old Inspiron 15R 5537 - latest wireless card?