I use Mac as well and never used Synology Assistant.
As suggested , you can try browsing to find.synology.com and that should provide information on all devices it finds, plus details of each one including the current IP address. Then you can use that IP address to browse to the NAS, not forgetting to add the HTTP/HTTPS port number. Or just click the Connect button on the device's page.
Your question about how to reserve an IP address for the NAS in your router isn't as simple as it seems. Every router has its own configuration interface and can use different terms to refer to the same type of setting. The assumption is that your router is assigning local IP addresses to your LAN devices, it's a reasonable assumption as most home users will be doing this.
If you log into your router you will be looking for the DHCP server settings. This will define a gateway IP (usually the router), a range of LAN IPs to use for client devices, DNS server IPs, and domain name (optional) that will be used to send network configuration settings to LAN devices. There should be a way to reserve IP addresses in the range of LAN IPs for specific devices: to do this you enter the device's MAC address of their Ethernet interface, then when it is used to request DHCP client information then the reserved IP is always sent back. An IPv4 MAC address is six alphanumeric pairs usually separated will a colon.
From the find.synology.com results you should see the MAC address of the discovered NAS. You can use this to reserve the currently assign LAN IP. Now your NAS will always be on this IP address, until you blitz the router
Regarding using Synology Drive on the DS215j, this NAS uses ext4 file system for its storage and that is not efficient when using versions in Drive Server. You may find that the versions database is very large and invisibly eating up your space. When you regain access to the NAS you should review the number of versions you need and also avoid versioning folders that contain huge amounts of active data.