2 Zyxel devices. VMG3625-T50B VDSL Modem and NWD6605 Wireless Adapter

Erman

Hello,

I am currently using 2 devices.VMG3625-T50B VDSL Modem and NWD6605 Wireless Adapter

The thing is, the wireless adapter is disconnecting or not connecting smoothly to the Modem at 5 GHz. 
There is no problem when the adapter settings in 2.4 GHz and can connect immediately. But when I change the working frequencies of the adapter to AC standards, the problem begins. 

In the beginning, Modem settings were all the same for 2.4 and 5 GHz. I separated them into 2 different SSID and passwords. 

Now, when I connect to 5 GHz SSID, It's all OK. Can connect immediately.
But in the same SSID with 2.4 and 5 GHz, the trouble begins.

The modem firmware is V5.50(ABPM.5)G0. Latest as they say
And the adapter driver is updated. 1030.41.514.2020

There is nothing to update on both sides now. 

The other thing is, Zyxel wireless adapter has a Realtek RTL8812AU chipset.
I have also Tp-link T9UH wireless adapter which has a Realtek RTL8814 AU chipset. 

There is no problem(disconnecting or not connecting) with the Tp-Link wireless adapter when connecting to the Zyxel modem with one SSID Or with two SSID separated to 2.4 and 5 GHz.

So no need for separating the SSID's for the TP-Link adapter. But need for Zyxel adapter.

Can we say, the Tp-link adapter is using the older chipset, and that's why this occurs?
Or, Are there really some software or firmware problems on the modem side?

By the way, the Zyxel changed the wireless adaptor with a new one. And I only found the solution to separate the SSID's. Too many tries for the other alternatives. 

This disconnecting or not connecting is a huge problem for many users and needed to be investigated. 

As you see I have written all the details that I have found.

Please take this into consideration. 
Thank you...