Most routers that support guest zone have 192.168.7.x restricted. Since the router intended for home use, it's unlikely that a consumer will use that particular subnet. Even if that's the case, a typical consumer may have 5-10 mostly dynamic devices (smart TV, ipad, smart phone, laptop etc). If they were to use the default 192.168.0.x subnet, they would just need to reboot all their devices once and voila! online.
Looking at your existing topology and assuming the whole network is 192.168.7.x and you may have printers, NAS's and other stuff all static it will be a pain to change everything to something else.
What you can do is change the LAN IP on your DIR-816L to say 192.168.0.55 (note it down), set all the wireless parameters as desired and then disable DHCP. Then plug the 816L via LAN port 1-4 to the network. The 816L will still act as an AP and function properly. The only downside is if you want to mange it, you will need to set a static on your PC to 192.168.0.xxx to log in. If your network is fairly static, you could in theory leave this operational indefinitely. If you like changing passphrases, channels, admin passwords on a weekly basis, this may be cumbersome and an Access Point would be recommended (DAP-2310).