No, it does not make sense.
When things like this happen to me, not necessarily with Blink alone, especially when support fails to help properly, I’m prepared to try a lot of things to find out what the problem is. This is my nature, and it helps being retired, thus having as much free time as I choose to allocate to it.
If it were me, I simply wouldn’t buy that a merged network is the issue. First, 5GHz isn’t even available in the sync module or the camera. Even if it’s on the chips, because it may not be possible to get chips without 5GHz now, it will not be turned on. This means that the Blink items can only see and do 2.4GHz things, so how can the presence of 5GHz have any impact?
Better, it has worked perfectly for you for years, as it has for me. I have a merged SSID network, and I have no issue at all with Blink, in this regard. Some IoT devices do have a problem, but this is because my phone is often connected to my 5GHz channel, and they tell me that I cannot set it up whilst I’m on 5GHz.
If this had been a problem to me, with Blink, I would have thought nothing of it, and simply disabled 5GHz for the duration of set-up, which is what I do for other IoT devices, and then turn it back on afterwards, always without issue. They too cannot see the 5GHz, and the router can’t see them, on the 5GHz band, so neither cares that it’s on. It’s only during set-up, when they talk to your phone, and they ‘panic’ if your phone is connected to 5GHz. Not that the phone would have any issue, when you have to connect directly to the device, because it would just switch to 2.4GHz, but still, the devices moan, for some reason. Blink however does not.
So, it works for me, and has for years. It worked for you, and has for years, and yet they blame your network. How can this be? Maybe your network changed, even unbeknown to you? A firmware update, or similar? Or, Blink support are giving you a bum steer.
Here’s how I would test it, and prove that the issue was account based or not. This is not without risk, in case the camera will not join back to your existing account, but I would still do it.
First delete a sync module and XT camera from your existing account. You now have a complete, unowned, system. Now, log out of your Blink app, and create a brand new account. Obviously you will need a real, viable, email account, that has not been registered with Blink before. Be sure that you know your old details too. Don’t want to get stuck out of the ‘old’ account.
Now, having created and logged in with the new account, go through the process of adding the sync module and then the camera.
If this ‘new’ system, on a new account, works perfectly, on your 2.4GHz/5GHz shared SSID network, you know support is wrong. Then, having got this evidence, call support, and ask them how it comes to be that the system is now working on the said same network?
Once they are convinced, reverse what you have done by first deleting the camera, sync module, and account. Then, you can add them back to your original account.
One final thing, having run the app, RouteThisHelps, with the support code BLNK, I can confirm that I do get a warning about shared SSIDs. The warning goes something like this. This is an advisory notice only that your network uses a shared SSID. This does not stop Blink from operating correctly, because it supports merged networks, and is advisory only, just so that you know that it is connecting to your 2.4GHz network only and cannot make use of your 5GHz band.
Not word perfect, but that’s the theme. It does go on to say, if you do have issues, you might try splitting the bands, but this still doesn’t make sense to me. If it is required, it’s a router problem with smart switching, not Blink’s issue, in my opinion.
Sorry to go on, but to me it would be worth a try, so that you can go back to them and say "oh, how come I’ve set it up on a new account, same network, and it’s working a treat?