HP OfficeJet Pro 8720 Wi-Fi connection issue on iMac

I just verified the following works ... at least it did with my HP printer.

We just need to modify the steps I provided you earlier. Please give the following a try:

1. Go to System Settings > Printers & Scanners
2. Click Add Printer, Scanner, or Fax
3. On the Add Printer window, select the IP icon.
4. Enter the IPv4 address of your HP printer. For example, mine is: 192.168.1.150
5. For Protocol, choose: AirPrint
6. For Name, enter the desired name so you can recognize the printer.
7. For Use, this should be your HP OfficeJet Pro 8720.
8. Select Add.

Your HP printer supports Apple's AirPrint. Since your iPhone is printing fine, I suggest starting by verifying that AirPrint is working on your iMac, let's start with with the following:

1. Go to  Apple Menu > System Settings > Printers & Scanners.
2. Click Add Printer, Scanner, or Fax (+ button).
3. Select your HP OfficeJet Pro 8720 from the list.
4. In the Use dropdown menu, choose AirPrint (instead of the HP driver).
5. Click Add and test printing.

M. Erik Husby wrote:

... When I reset the printer network settings it complained that there was more than one access point/wireless router found with the same SSID which could cause confusion for the printer ...

Yep, that would be the reason. Great troubleshooting, and I'm glad you got it working.

Another thing, if you have done so already, is to configure your printer with a static local IP address, outside of the DHCP range provided by your Orbi router. That way it will be "easier" for network clients to "find" the printer.

So, for example, if your Orbi gives out local IP addresses in the 192.168.1.10 to 192.168.1.150 range, configure your printer to use 192.168.1.151 for its IP address.