Hi Ko,
I have had a similar issue with my XPS-13. To help resolve this issue and because I move my laptop between two locations, with different external monitors, I wrote this script and assigned it to the F8 key so that I can easily toggle between the two locations without having to rely on the UI. Maybe you can use it, maybe in an edited form, to help you resolve your issue, too.
toggle-1080p.sh
#!/bin/sh
# toggle-1080p.sh script
#
# Toggles between 1080P and 4K
#
# This is the version for Ubuntu MATE on the Dell XPS13 9360
# eDP-1 = laptop | DP-1 = Hisense | TV DP-1-1 = Samsung
# ##########################################################
# Larry Bushey
#
mode="$(xrandr -q|grep -A1 "DP-1-1 connected"| tail -1 |awk '{ print $1 }')"
if [ -n "$mode" ]; then
# hidpi="$(xrandr|grep "eDP-1 connected primary 3200x1800")"
hidpi="$(xrandr|grep "DP-1-1 connected primary 3840x2160")"
if [ -n "$hidpi" ]; then
# xrandr --output eDP-1 --left-of DP-1-1 --mode 1920x1080
xrandr --output eDP-1 --off
xrandr --output DP-1-1 --mode 1920x1080
gsettings set org.mate.interface window-scaling-factor 1
notify-send --icon=/home/lebushey/bin/icons/gnome-display-properties.svg "1080p" "Your display is now set to 1920x1080 resolution."
else
# xrandr --output eDP-1 --left-of DP-1-1 --mode 3200x1800
xrandr --output eDP-1 --off
xrandr --output DP-1-1 --mode 3840x2160
gsettings set org.mate.interface window-scaling-factor 2
notify-send --icon=/home/lebushey/bin/icons/gnome-display-properties.svg "HiDPI" "Your display his now set to 3200x1800 resolution."
fi
else
hidpi="$(xrandr|grep "eDP-1 connected primary 3200x1800")"
if [ -n "$hidpi" ]; then
xrandr --output eDP-1 --left-of DP-1 --mode 1920x1080
xrandr --output eDP-1 --mode 1920x1080
gsettings set org.mate.interface window-scaling-factor 1
notify-send --icon=/home/lebushey/bin/icons/gnome-display-properties.svg "1080p" "Your display is now set to 1920x1080 resolution."
else
xrandr --output eDP-1 --left-of DP-1 --mode 1920x1080
xrandr --output eDP-1 --mode 3200x1800
gsettings set org.mate.interface window-scaling-factor 2
notify-send --icon=/home/lebushey/bin/icons/gnome-display-properties.svg "HiDPI" "Your XPS-13 display his now set to 3200x1800 resolution."
fi
fi