matteovisco It sounds like youre having an issue activating the matvt mouse with the UI, so try installing the MATVT mouse, but activating it with adb [or webadb shell] use these commands
adb shell appops set com.android.cts.io.github.virresh.matvt SYSTEM_ALERT_WINDOW allow
adb shell settings put secure accessibility_enabled 1
adb shell settings put secure enabled_accessibility_services com.android.cts.io.github.virresh.matvt/com.android.cts.io.github.virresh.matvt.services.MouseEventService
adb shell am startservice com.android.cts.io.github.virresh.matvt/com.android.cts.io.github.virresh.matvt.services.MouseEventService
If youre using WebADB shell, leave out "adb shell" from the commands