I have a 1100AHx4 (Dude) and a wAP ac connected as an AccessPoint.

Everything is working but I notice that all my device (mobile and computer) that connect to the wifi dont use 5GHz band just 2.4GHz.
I have same SSID for both 2.4 and 5 GHz band.
What have I missed and what more information do you need from me?

I want my devices to change from 2.4GHz and 5GHz seamless.

Try to have different ssid. Check that you use standard channel.

Change SSID on 5GHz but dont see it in the phone.
Strange I think I did this before and it worked.

I noticed similar behavior in several occasions:
- 5G Wifi not visible at all - caused by weird Realtek wifi module in ThinkPad E580 which have very limited range of 5G frequencies. All AP frequencies (control and all extensions) were out of supported range
- 5G Wifi visible but not connectable - again caused by same Realtek module. Some AP frequencies were in supported range, some were not. Realtek module saw wifi, but refused to connect.
- 5G wifi visible, connectable, but devices prefer 2G - saw this several times. This is up to driver/OS to choose faster (not stronger) channel. If device cannot prefer 5G over 2G, you will find them connected to 2G all the time, unless you manually reconnect them to 5G. AP can theoretically kick them from 2G by either ACL or script, but that is just poor patch of the real issue.

Maybe your cause is same? Hopefully this will help.

Same issue here - I am not convinced of 5G at all!
Reason is - from my not amateur perspective - that wap ac 2G power is higher than 5G power and furthermore 2G makes it so much better through walls than 5G.
In fact 5G is working only in sight distance at my home. As soon as any walls come into play (even at low distances like 10m) my devices work only at 2G reasonably.

But that is just basic physical laws. 5GHz waves are twice as short, therefore much more affected by obstacles. Unless you manually limit power of 2GHz wifi, you will always end up with 2Ghz wifi being stronger than 5GHz. It is clients responsibility to choose properly. For example every proper Intel Wifi module has option to "prefer 5Ghz" over 2GHz. Many mobile phones which I saw connected to my AP do the same - automatically prefer 5GHz unless the signal is really poor.

It is really not fault of your Mikrotik.

5ghz does have a lot shorter range than 2,4ghz. this is wlan theory laws

Remember to use correct channel. I dont know about your chipset, but some chipset might just use some channels and not all avalible in mikrotik. Never use superchannel, unless you know 100% what you are doing. use correct country code, in some cases not your country, but whats on your client chipset driver.

Same issue here - I am not convinced of 5G at all!
Reason is - from my not amateur perspective - that wap ac 2G power is higher than 5G power and furthermore 2G makes it so much better through walls than 5G.
In fact 5G is working only in sight distance at my home. As soon as any walls come into play (even at low distances like 10m) my devices work only at 2G reasonably.

One of the things I like about 5GHz is EXACTLY the wall attenuation. I hate to look at some polluted nightmare, seeing neighbors' wifi 5 floors below. Much better to use more APs, with lower TX power, than the opposite.