Do you have an Intel processor with integrated graphics? If so, using it
would certainly provide you a correct resolution and better performance. And
physically removing the AMD card would save energy.
If not, try a newer kernel. Trisquel 8 ships with Linux 4.4 but you can
install, from its repository, the package "linux-image-generic-hwe-8.0" to
get a 4.15 image. You can have version 4.19 or even 5.2 following the
instructions on https://jxself.org/linux-libre/
You need to reboot on the newly installed kernel (the newest version is the
default) before trying again to change the resolution.
