@Kevin_Z 

THX for your feedback.

My network based on Deco E4R (Local IP 192.168.0.1) and M3W (DHCP IP 192.168.0.101). I have two same nootebooks (Dell N7110) which only work with 2.4GHz wifi network. I have adapter Archer T2U Nano. When i'm use adapter, different problems occurs in 2.4 and 5 GHz network. In 2.4GHz network I have small download speed (~3Mbit), but upload speed is 80Mbit. In 5GHz network I have periodically very hight ping and lost packages. I'm thinking about this and maybe problem with T2U adapter. BUT this adapter works whitout described problems with other Router (Archer C20) in 2.4 and 5 GHz networks

Now I don't have records in log with error described in my first post. But have several new one 

Wed Oct 16 22:26:50 2019 daemon.err uhttpd[4237]: ath22 no frequency information.

Wed Oct 16 22:26:50 2019 daemon.err uhttpd[4237]:

Wed Oct 16 22:26:55 2019 daemon.err uhttpd[4237]: ath22 no frequency information.

Wed Oct 16 22:26:55 2019 daemon.err uhttpd[4237]:

Wed Oct 16 22:27:08 2019 daemon.err uhttpd[4237]: ath22 no frequency information.

Wed Oct 16 22:27:08 2019 daemon.err uhttpd[4237]:

Wed Oct 16 22:28:57 2019 daemon.err udhcpd[9597]: Sending ACK to 192.168.0.101

Wed Oct 16 22:28:57 2019 daemon.err udhcpd[9597]: clear ip c0a80065

Wed Oct 16 22:28:58 2019 user.warn IMPROXY: WARNING[mcast_recv_igmp@753]: Don't exist this Downstream VIF

I tested ping status with programm PingPlotter:

I want change wifi chanel, but Deco app dont have this function. And this problem occurs time to time. If you can help me, it's will be good...

Sorry for my English. With best regards from Ukraine

@Kevin_Z 

Thanks for your reply.

"For the speed issue of 2.4GHz, do the 2 laptops have the same issue?" - Have problem on 2 laptops only on 5GHz network

"With them connected to the 2.4GHz of the Deco network, please click here to verify the wireless negotiation speed." - verifing speed on 2.4GHz on 2 laptops (build-in wifi) is 150Mbit

"And how far away between them, what about the wireless signal strength"

"For the latency issue, how many 5GHz networks can be detected via the Archer T2U nano?" - Only my network. BUT in wifi network list i see one network with hidden name. And wifi signal strength is similiar to my network. It's strange. And this network appear when I turn on my Deco.

"What is the version of the driver you installed?" - Driver version T2U Nano is 1030.29.329.2019 dd 12.04.2019 (TP-Link Technologies Co., Ltd.). I try another drivers (TP-Link and Microsoft), but problem is no gone.

"Do you have any other devices which support 5GHz can be used to do a test? " - Only cell phone Samsung S8. Test ping with "PingTools" app. Ping is normal and have download speed better

Thanks for you reply.

@Kevin_Z 

Link with driver is not work: "file does not exist"

@Kevin_Z 

Install driver for T2U Nano from your link - v1030.29.1102.2017 dd 18.07.2018 (TP-Link Technologies Co., Ltd.).

Bandwidth from internet operator is 100 Mbps download/upload. 

From laptop with built-in wifi card (2.4Ghz)

Link

From laptop with T2U Nano (2.4Ghz). Very unstable conection. Adapter T2U Nano some times goes down.

Link

From laptop with T2U Nano (5Ghz)

Link

Ping from laptop with T2U Nano (5Ghz) to Deco

And in Wi-Fi network list I have several hidden network, which similiar to my 2.4 and 5GHz network (phpdes.com). It's strange

@Kevin_Z 

Thanks for your reply.

I will contact my the retailer for change adapter to another.

Several words about hidden networks. They can be detected by T2U Nano (2.4GHz and 5GHz) and laptop built-in wifi card (2.4GHz). But on phone (Samsung S8) cannot be detected.