I have tried the simple steps detailed by @HarvsG by installing the 4 Tailscale plug-ins (just make sure that all 4 packages are installed before rebooting the router) to get Tailscale on my GL-AR750S.
Tailscale installed perfectly well on the AR750S but I could not easily get the binding link to connect the device to my Tailscale network until I started experimenting with different snapshot 4.x.x. firmware versions. Managed to get the binding link on both v4.3.2 and also 4.3.6 which I have kept installed on the router for now although I could not get the binding link to show up for love or money on the latest 4.3.7 available snapshot. Anyone knows why it worked with the slightly older versions of the snapshot but not on the latest one and whether there is a way to fix this?
Hope this helps you get it working on your Opal as well.