2nd Monitor Randomly Isn't Detected

Question

This question may seem like a duplicate, but there are a few key differences.

xrandr only mentions my laptop's internal monitor:

Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 8192 x 8192
eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 344mm x 194mm
   960x540       59.82  
   864x486       59.92    59.57  
   640x480       59.94  
   480x270       59.82  
   432x243       59.92    59.57  
   320x240       60.05  
   1920x1080_60.00  59.96*

When I plug in the HDMI cable, my external monitor wakes from sleep, but then goes back to sleep. There's no other indication that my computer is communicating with the monitor.

I deleted my xorg.conf to no avail. In fact, my primary, internal monitor started using 960x540 when I rebooted.

Solutions I've tried:

• Rebooting
• Shutting down and turning back on
• Deleting xorg.conf
• Repeatedly unplugging and replugging my monitor's cable on both ends
• Various console solutions
• And more...

Please help. I'm running out of options.

Edit: Hardware is as follows

• Inspiron 5558 Laptop
  • 16 GB RAM
  • Intel UHD Graphics 620
  • 512 GB SSD
• Ubuntu 18.04
  • No idea what graphics driver (can't find it, Additional Drivers application is non-existent)

Edit: output of lshw -numeric -c video

*-display                 
       description: VGA compatible controller
       product: UHD Graphics 620 [8086:5917]
       vendor: Intel Corporation [8086]
       physical id: 2
       bus info: pci@0000:00:02.0
       version: 07
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
       configuration: driver=i915 latency=0
       resources: irq:145 memory:d0000000-d0ffffff memory:c0000000-cfffffff ioport:f000(size=64) memory:c0000-dffff