Dell 3520, can you add 5g wifi

Intel Centrino Advanced-N 6200 works and has 5G wifi. I've tried a number of wireless cards in mine and that one works great in 5G, but not quite as well on older standards.

Yes, absolutely.  2.4 GHz and 5 GHz use the same types of antennas, so you'd just need to pick a card that has the same number of antenna connectors as the system has antennas built into it, and the same motherboard connector as the card you already have, e.g. mPCI, mPCIe, or M.2.  Unfortunately I can't tell you anything about your system because you just said "Dell 3520" without specifying the model line, and there is both an Inspiron 3520 and a Precision 3520.