Lot of comments but the real answer is that generally router firmware is pretty shitty and price does not equal performance. If you want something good for cheap get a 50-100 dollar (can find even cheaper especially if you don’t have crazy internet speeds) computer (like a raspberry pie clone) stick on open wrt, and get a separate Wi-Fi access point. Will blow past any consumer grade premium router because a lot of them are sold the be “all in one” with the cheapest, low power hardware imaginable and while not universal, software that never updated. Also, Something like openwrt has something called cake sqm which can massively reduce latency as well as constant software updates that fix vulnerabilities.

You need to set the scale in the sddm config file. I had the same issue where Kde properly set the theme but not the scaling. Here is a guide from the arch wiki

https://wiki.archlinux.org/title/SDDM#Enable_HiDPI

To find your screen dpi I think there is something in KDE display settings.