Ubuntu 16.04.3 LTS + lg-29UM58-P (ultrawide 21:9 2560x1080)

When I set the update frequency to 50Hz, the monitor started working correctly.

Steps:

First, check the options to be set given the resolution and frequency:

gtf 2560 1080 50

It will output something like:

# 2560x1080 @ 50.00 Hz (GTF) hsync: 55.60 kHz; pclk: 188.60 MHz
  Modeline "2560x1080_50.00"  188.60  2560 2704 2976 3392  1080 1081 1084 1112  -HSync +Vsync

Then, use this information to create a new xrandr mode:

xrandr --newmode "TEST" 188.60  2560 2704 2976 3392  1080 1081 1084 1112  -HSync +Vsync
xrandr --addmode HDMI-3 "TEST"
xrandr --output HDMI-3 --mode "TEST"

You can also add a script to autoload this configuration at system boot time. I got the script from Script for autoloading resolution. You can create the file ~/.xprofile with the content (replace where needed):

#!/bin/sh
MODE_NAME="ULTRAWIDE"
OUTPUT="HDMI-3"
CONNECTED_OUTPUT=$(xrandr --current | grep -i $OUTPUT | cut -f2 -d' ')

if [ "$CONNECTED_OUTPUT" == "connected" ]; then
  # SET 2560x1080 50 HZ
  xrandr --newmode "$MODE_NAME" 188.60 2560 2704 2976 3392 1080 1081 1084 1112 -HSync +Vsync
  xrandr --addmode "$OUTPUT" "$MODE_NAME"
else
  echo "ULTRAWIDE IS NOT DETECTED !"
fi