t has both bands on the same SSID
Use different SSIDs for the 2,4 and 5 GHz band in the router. That´s the easy way to solve it. I do it this way. But maybe there is another way.
How do I force the Rpi to connect to the 5Ghz band and only fall back to 2.4 when theres a problem?
Technically you can't as this will be router dependant, the router is probably 802.11n and not ac, the Pi has dual band 802.11ac WiFi so to connect and maintain a 5Ghz connection you'll be restricted by the distance and obstacles between the Pi and the router.
I assume by your statement above that you were experiencing drop outs on the 5Ghz channel, you could try changing the channel number but that usually doesn't help, moving the Pi or the router closer together without obstacles in the way.
How do I force the Rpi to connect to the 5Ghz band and only fall back to 2.4 when theres a problem?
I think that is the default behavior.
There is a problem and that is why 2.4 is used. The problem, most likely, is that the strength of the 5 GHz connection is too low or is disturbed.
This can be fixed in a number of ways.
Change the orientation of the antennas and/or devices.
Upgrade to better antennas.
Reduce the distance.
Use another channel.
Make a hole in the wall(s) between the devices.
Make a hole in the floor(s) between the devices.
The way I did it was to combine several of the above methods by using a wifi mesh system, with several nodes, instead of a single router with wifi.
I use ASUS Lyra, works fine. Two relatives are neighbors to each other and share wifi (and an OMV HC1 I set up). It barely worked, except in one or two rooms. I bought and installed a 6 node Tenda MW6 wifi mesh there. 2x3. It was an amazing improvement.
I also have a DIY 2.4 GHz "mesh" consisting of a bunch of small Gl-inet mini-routers configured as repeaters. I use this 2.4 GHz fake mesh mostly for iot stuff and cameras, but it is bridged to the 5GHz mesh as well. I have a farm house with several outbuildings. The 2.4 GHz fake mesh gives great coverage. But with low speed and high latency.
Make a hole in the wall(s) between the devices.
Make a hole in the floor(s) between the devices.
That´s for sure, but which tools should be used for that? A jackhammer? 
Unfortunately the women acceptance factor is limited.
That´s for sure, but which tools should be used for that? A jackhammer?
I typically use a chainsaw to make holes in the walls and floors. Wooden house.
For my protection I also put on a white hockey goal keeper face mask.
I live alone. 
the router is an asus ac86u so its ac and should be able to do 5ghz.
According to the specs it does, so the problem is down to the Pi, I found something on the Pi forum that might help as the user that maintained the Pi image is no longer here so I have no idea if the forum solution could be used.
maintained the Pi image
With OMV 5.x, the official Raspbian Buster Lite image should be used with the install script. Once a user is using that image, just about any suggestion on the RPi forum should work. In my opinion, all RPi users should be using that over the OMV 4.x image.
all RPi users should be using that over the OMV 4.x image.
I would agree as I have tested OMV5 using your script on a 3B+ and it worked flawlessly 
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