Disconnection issue in Mikrotik LHG LTE6

Devices involved:

  1.  Mikrotik Router (LHG LTE6) - outdoor unit (Main Router)
    

Current firmware version: 6.48.4
Router OS version: 7.2rc1 (testing)
2. D-Link Dir 615 N300 router- indoor unit (acts as an Access Point)

Connection Setup details:

 Mikrotik Router is connected to D-Link (Which acts as an Access point)
 IP address is assigned by the main router which is Mikrotik
 So, all the devices connected to the D-Link Wi-Fi will get IP address from the main router (Mikrotik)

Setup diagram: PFA

Issue description:
 Problem I’m facing here is, suddenly all the devices connected to the D-Link Wi-Fi get disconnected, even the D-Link also gets disconnected from the
Mikrotik router.
 Error message in the smartphone: “obtaining IP address from the DHCP server” and disconnects and not even connecting to the WIFI
 Error message in the D-Link: Failed to obtain IP address from the DHCP server

Troubleshooting done to isolate the issue:
 Once the Wi-Fi disconnects or the issue occurs, I connect my laptop directly to the Mikrotik router and it says “unidentified network”
 Not even able to ping the device. (Mikrotik: 192.168.88.1)
 In ‘stable’ and ‘long term stable’ versions I also face the same problem. So, I upgraded to the testing version and there also I’m facing the same problem.
 Checked the D-Link with router mode also, facing the same issue.
 Even if the link goes down due to the telecom service provider which I’m using, it should reconnect after a minute right, but it is trying to obtain the IP address from the DHCP server for more than one hour. (Only after switching off and then on, it working)- mentioned detailly in the temporary solution section.
 Even tried rebooting the Mikrotik router but no use.
 From the above things I confirmed that the problem is with the Mikrotik router.
Note: Bought new Router TP-Link AC1200 Archer C6 and tested the same but the issue remains.

Temporary solution:
 I just switch off all the device and switch it on after one minute and it’s working fine.
 Again, after certain times the problem appears, and I will repeat the same to solve the issue.

Queries:
 Why did the Mikrotik router suddenly de-assign IP addresses to the devices connected to it including the Access Point(D-Link)?
 Why does the problem solve only after switching it OFF and then ON?
 Are there any configurations that need to be changed in the router? If yes, tell me what to do.
 I’m ready to provide any information you need. My priority is to get the problem solved.

Note:
 If you are suggesting a solution, please explain clearly and step by step.

I hope you guys will help me in resolving the issue.
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Hi, did you solve it? I have your identical problem with Lgh kit18 version. Thanks