My monitor is ViewSonic VG2030wm and it is capable of working at 1680x1050 resolution but Xorg can't detect that and highest resolution that I can use is 1024x768.
You need to add ModeLine so the Xorg can recognize it. Here is my Monitor section from xorg.conf:
Section "Monitor"
Identifier "ViewSonic"
VendorName "ViewSonic"
ModelName "VG2030wm"
HorizSync 24.0 - 92.0
VertRefresh 50.0 - 85.0
Option "DPMS"
ModeLine "1680x1050" 170.00 1680 1784 1960 2240 1050 1053 1059 1089 +hsync +vsync
EndSection
Probably you will need just the ModeLine row but I am publishing the whole Monitor section.
You can find more ModeLine for certain monitors here: https://www.mythtv.org/wiki/Modeline_Database