It doesn't matter in that the disk doesn't have to be formatted HFS+ for your Mac to be able to use it if the router's sharing it as a Windows file share - as phrehdd says, as long as the router can read and write to the disk you're good to go.
Unfortunately some routers do NOT support HFS+ (like my current Archer C7). So, yes, I can connect it to the router and share it on the network, but cannot see anything on it. The Router presents my 2TB Seagate drive as an empty 196MB folder, and my 4TB Cineraid enclosure as an empty 206MB empty folder. TP-Link has confirmed that their USB driver in their routers does not support HFS+ formatted drives. If I connect my NTFS drive (I still have 1 of those) it works as expected. Have been looking at other routers, and this looks like it is more common than not.
For me, the bigger problem with the A C7 is that it is limited to 2 USB2 ports. USB2 might be fine for a printer but certainly not for adding an external hard drive which should be connected via USB3.
The goal for adding external drives would be to find the fastest combination with the least negative impact. This means a good performing external drive, connect via USB3, a router that has a good reputation for handling access to the external drive along with requested reads and writes without impacting other connectivity (WiFi and Ethernet). I think the A C7 is a nice utility router. USB2 for printers and emergency, lackluster handling of 5grtz (it suffers when signals are too strong) but works well and is faithful based on more serious technical reviews along with some comments by owners.