I sure hope you have a remedy already. Just to explain, in the default mode the Sky Hub broadcasts both 5GHz and 2.4GHz at once under a single network name.
Historically, because 2.4GHz travels further distance as it penetrates materials better (e.g. bricks / concrete etc), devices would almost always go into 2.4GHz when 5GHz was named the same, because devices choose the network with strongest signal. Whilst 2.4GHz travels further, it's an older technology that is typically around 10x slower than the theoretical max speed of 5GHz. Hence, people often had different names for 2.4GHz and 5GHz so they could ensure 5GHz connection.
Since SkyQ launch and the Hub 3 where Airties was deployed for the SkyQ mesh, the routers have been smart enough to mitigate this via Band Steering. When a device connects to the Sky hub it indicates capability e.g. it says "I can do 5GHz and 2.4GHz and I have good signal." In this case, the Sky Hub blocks the device entering the 2.4GHz signal even if it tries, this forces devices into 5GHz. If after a period of time, these devices do not show up on 5GHz, indeed the Sky Hub will let them into 2.4GHz (eventually). If signal becomes poor on 5GHz, the Sky Hub will handle a transition to 2.4GHz, and handhold the device as it moves over.
In the case of your Arlo cameras, they are 2.4GHz only. Hence they should begin connecting to the Sky Hub, and indicate they are 2.4GHz only devices. Here, the Sky Hub will allow them into the 2.4GHz network no problem, always and forever. There should be no reason for 5GHz to be disabled.
What seems to have happened here is the Arlo app performs a check on the mobile that looks at the network name, and wireless band the phone is connected too. If the phones on 5GHz, the Arlo app stops you continuing because the Arlo cameras it thinks cannot connect to this 5GHz network. This is the most dumb and illogical programming, as it basically ignores any notion of "dual band" networks. Due to the prevelance of band steering, 8 in 10 devices in use today, that are issued by UK ISPs are dual band with band steering. They all have a single network name, operating on both bands at once. The last major device issued by the most prevelant major mass market ISPs that split 2.4 and 5GHz was the Virgin Superhub 2 AC released 2015 (9 years ago).
If the Arlo just took the network name and password and tried to connect, it would do so just fine. There should be no requirement for your phone to be on 2.4GHz, for the Arlo to process and try a network name / password. It's ridiculous.
Anyway, if the sky portal is not providing ability to re-enable 5GHz band - holding the reset on the back of the sky hub should revert it to initial setup. Assuming you are using default name/PW, everything will connect back just fine (even the Arlo's as they're now in receipt of the network name and PW). If you're able, raise feedback into Arlo they are a big US company. This should not be how their app logic is programmed.
