I just bought a Raspberry Pi 4B with case, micro SD, and power supply. Configured it to connect to my 5GHz SSID and happily started to renew my acquaintance with the command line.
Using Imager 1.7.3 I configured a new micro SD card with the latest OctoPi Stable release, 0.18.0 with OctoPrint 1.8.6 and tore my hair out trying to connect to my 5GHz SSID.
When I changed the SSID to my 2.4GHz one, OctoPi grabbed a DHCP address and notified me of it.
Is there the intention of adding 5GHz access to OctoPi going forward, or is this left to the user?
Just to clarify - OctoPrint itself relies on the network connection of the OS.
So if you can't connect to your network the OS is to blame.
I'm also using the latest OctoPi stable image and 5GHz wifi works fine.
There could be multiple reasons why you can't connect to you 5GHz network like bad reception, wrong ssid or password (sometimes there is a hidden space in the ssid), wrong wifi country, bad power supply and of course it could be because of a router / access point setting like mac address restriction.
Where did you enter your credentials?
Are you familiar with ssh?
I would add a suggestion to test it with a regular RPi OS image as well - that's what OctoPi is, just with preinstalled OctoPrint - so if that doesn't work either it's definitely not anything OctoPi-specific.
Sorry, that was the very first line of my note.
OK, have a Pi 4B, monitor, keyboard, mouse, Pi power supply,
using Imager to create a Pi OS image with 5GHz SSID and pass and country, entered through Imager, Pi works fine.
using Imager to create a OctoPi image with 5GHz SSID and pass and country, entered through Imager, no IP on OctoPi screen on monitor.
using Imager to create a OctoPi image with 2.4GHz SSID and pass and country, entered through Imager, DHCP IP on OctoPi screen on monitor.
Having done tech support all my working life I copied and pasted SSID and Password, attempting to reduce variables. The Pi was in the same place, just shutdown, removed power, replaced micro SD card powered back on.
So this was why I asked the question, using Imager to create one system works, the other does not.
If everyone else can make this work, I can just shrug and walk away, but as I can repeat the issue thought I would ask.
Ok that rules out a lot of stuff 
The main difference between the Pi OS image and the OctoPi stable image is that the Pi OS is based on Debian Bullseye while OctoPi stable is still on Debian Buster.
In theory both should work, but if that's the issue you could try it with the OctoPi nightly image which is also based on Bullseye.
I hope that solves your issue 
Thank you for that suggestion, I was struggling to work out what the differences were. Have not done much Linux/Unix in the last 20 years so am a bit rusty. I have been spoiled with Windows/Mac GUI's to find stuff, and cudgelling my brain to remember what things are called to access the man pages has been a challenge.
I'm using octopi 1.0.0 RC3 which is bullseye based and that works (for me at least) with Wifi 5G.