(Oh, LuCI is the name of the OpenWrt Linux GUI. GL builds on OpenWrt. LuCI can be accessed by GL GUI → System → Advanced Setting; same password as the GL GUI, username root . Don’t use it in this case. 
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Can you confirm :51820 is open|filtered when the Opal is hard wired to the modem? I’d really take Wi-Fi out of the equation for the moment. That IP Void link should be able to confirm it.
Don’t worry about that; if anything you should try setting your Slate AX’s IP to 192.168.18.1*, then connect its WAN to the Opal’s LAN. Generate new confs on your Opal (acting as the WG Server) & connect to the Opal to confirm WG Client/Server works properly when directly linking the GL devices. Taking the Verizon modem out of the equation will at least give you piece of mind it’s not the GL devices fight’n 'ya.
Here’s a brief HOW-TO/synopsis for what this looks like:
But to answer your question: 10.20.0.0/24 is a network subnet/IP range of 10.20.0.1–10.20.0.255. Such 10. addressing is typically used for VPN IPs as is this case. A ‘stock’/default GL device will have a LAN subnet of 192.168.8.0/24 (192.168.8.1–192.168.8.255).
If you connect your Slate AX & Opal to the same network in their stock configuration, gremlins are summoned so it’s best to set one of them to something like 192.168.18.1.*
Failing a directly connect WG Client/Server setup the only other thing to inquire is if Verizon is using CG-NAT. If so, WG is probably a hard no go. CG-NAT basically means there’s another ‘router’ on the ISP’s network that they may/may not open :51820 on their end to let you access your Public/Internet IP… which is need for WG when travelling.
* GL GUI → Network → LAN → Private Network → Router IP Address