Apple Watch Series 5 "Unable to check for update"

When attempting to update on the watch directly, it says "Unable to check for update. An error occurred while checking for a software update".

When attempting to update it from the Watch app on my iPhone, it says "you first need to update the software on your iPhone", but I'm running the newest iOS. There's no update.

I've tried resetting both devices, formatting the watch completely, unpairing and pairing again, etc etc.