2023 update:
Found an article with all kinds of information about this device, including detailed instructions on how to use the Plume app to change it to bridge mode. It’s buried pretty deep in the article, but it is there.
Here you can also select router or bridge mode and configure DNS and Port Forwarding settings.
It’s possible that the plume app navigation has changed since that article was written, in which case, again, you will need to contact plume support. But at least you know that it should be there somewhere. 