So now that’ve I got a touch of coffee into me I see this is also a question of Tailscale… which is still a ‘beta’ supported feature.
It might not seem related but on other (stable) firmware the tethering interface gets a lower metric than Ethernet/WAN so it can cause unforeseen conflicts. I read your Quectel EC25G modem as such a device. I may be mistaken. The first thing I’d check is that everything runs as expected using said modem before adding on Tailscale.
Regarding Tailscale, which I personally don’t use, I’ve cooked up a workaround three days ago that’s been reported to work as expected. The Slate Plus & the Flint are similar enough any differences shouldn’t matter. It’ll require some editing of the stock GL script but it’s hardly complicated. Perhaps you can template from it.
(Though I would stay away from beta firmware unless there’s an absolute impasse on stable. Beta is beta for a reason, after all.)