Switching Wallpapers Laptop + 21:9 Monitor

Hi guys,

I have a (probably dumb) question.

I have a 14" Huawei Matebook with a screen Resolution of 1920x1080.
I also own a monitor with 2560x1080 resolution.

Now, when Iḿ at home I use the 21:9 as a main screen. I´d like to have the same Wallpaper on both screens. Thats no problem. I just downloaded the same wallpaper with different resolutions.

Now, everything is fine when the Laptop is connected to the monitor. But when I remove the HDMI cable from my Laptop, I have the 2560x1080 Wallpaper on my Laptop which is scaled.

How do I solve this? That I always have the 1920x1080 Wallpaper on my Laptop regardeless if it is the main screen or not?

Thank you in advance.