External monitor is no longer detected after iMac has been in sleep mode

Hello,

I have an iMac (27-inch, Late 2013) running macOS Catalina (v 10.15.7). I've started using a secondary monitor (a ViewSonic), connected by an HDMI to Thunderbolt cable.

The second monitor works perfectly after I reboot my iMac, but if my iMac display goes to sleep, the second monitor is no longer detected. The second monitor's display stays black and my iMac's display flashes black (until I disconnect the HDMI to Thunderbolt cable).

I've already tried the following:

• Disconnecting and reconnecting the HDMI to Thunderbolt cable (both ends)
• Turned external monitor on and off
• Disconnecting and reconnecting secondary monitor from outlet (after leaving it unplugged for a few minutes)
• Trying to "Detect Displays" in System Preferences -> Displays

Any help would be much appreciated.

Thanks!