External Monitor not detected

Hello everyone

I would like to preface this by saying that I'm new to Linux, so any help would be appreciated. I googled this problem before posting, and I've tried the solutions provided in other threads/forums, but to no avail. I have an external monitor I would like to use, but there's no signal output, and it isn't being detected either. The monitor works fine, I've tried it on my other PC. I'm running Xubuntu 18.04 at the moment, and my xrandr looks as follows:

xrandr: Failed to get size of gamma for output default
Screen 0: minimum 1366 x 768, current 1366 x 768, maximum 1366 x 768
default connected 1366x768+0+0 0mm x 0mm
1366x768 76.00*

This is with the monitor plugged in via an HDMI cable. My aplay -l is as follows:

**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: ALC269VC Analog [ALC269VC Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0]
Subdevices: 1/1
Subdevice #0: subdevice #0

I'm guessing from this that my HDMI port is still functional.
sudo lshw -C display output as follows:

*-display UNCLAIMED
description: VGA compatible controller
product: Thames [Radeon HD 7550M/7570M/7650M]
vendor: Advanced Micro Devices, Inc. [AMD/ATI]
physical id: 0
bus info: pci@0000:01:00.0
version: 00
width: 64 bits
clock: 33MHz
capabilities: pm pciexpress msi vga_controller bus_master cap_list
configuration: latency=0
resources: memory:e0000000-efffffff memory:f7e20000-f7e3ffff ioport:e000(size=256) memory:c0000-dffff

I've also tried switching between Lightdm and gdm3, still no output.
Please let me know if any further details required.
Any help would be greatly appreciated. Thanks in advance.