In system settings -> Display there is "Scale Display" button which opens new window where you are able to set scaling of on-screen elements and text. However, it is not possible to set scaling individually per display. I wish it to be possible, because it will be more convinient to work with several displays with different DPI.
Thanks for the bugreport! David Edmundson has been working on supporting different dpi values per display, (at least for Wayland, on X11, that's going to be a lost cause), but this feature isn't available to the user yet. We're working on it.
Seems to be related to https://bugs.kde.org/show_bug.cgi?id=222428 ? I also wish the were a possibility to specify different DPI per display, I guess it's becoming a more general situation when people have HiDPI screens on laptops and some lower DPI external monitors.
Is there a workaround for this in Wayland?
No, it needs to be implemented. We're working on it though. I'll mark this as dupe of the original. *** This bug has been marked as a duplicate of bug 222428 ***