No software needed. Just share the Neo's external drive on your home network using the sharing feature built into Mac OS. Then, on your main desktop, point TM to the Neo's shared drive. (Edit: This assumes you don't already have a second Mac permanently on your local network. Something like a "shared Mac" for guests or kids to use? If you do, then congrats! - there's no need to buy a Neo!)
Of course, you'll need to leave the Neo (or other Mac) powered on 24x7 (along with its external hard drive) if you want to emulate exactly what the old time capsule offered.
OTOH, buying a cheap (~$200) NAS - like an Ubiquiti UNAS2 or a UGREEN DH2300 and sticking a single hard drive in it (which you could easily pull out of an old time capsule for free, if that's what you have now) would be cheaper and simpler. My original suggestion to use Neo to replace a TC was made in retrospect: It was made AFTER I'd purchased an expensive NAS (Synology) and filled it with expensive hard drives (~$1.6K in total). A Neo and a big external hard drive would've been half the price.
