@Sathiya 

Have you tried to connect this USB modem to your laptop directly and test if it is stable?

Is there any Link Backup settings you've done? Try remove all link backup settings. (WAN port will turn to Down according to link backup settings)

  @Somnus Yes, i tested the modem with my laptop and it is stable. As you can see in the screenshot, it is not specific to the USB modem, other WAN connections also reconnects but that's not frequent as USB modem. 

Only load balancing enabled, no link backups. I forget to mention in the initial message that I'm using the router with Omada. 

It turned out that the cable that I used was faulty. I used the simple ping command to find out. I connected the hotspot with a different cable to the laptop and pinged the Google DNS server, which didn't disconnect. I switched to the faulty cable, repeated the steps, and observed that the connection was often interrupted. I connected the working cable to the router, which has been running without disconnects for more than 4 hours now. Let's see. 

  @Sathiya 

I've had the same issues since using the ER605 V2.
The network disturbances were unbearable and as long as there is no error-free firmware I removed the ER605 from my network.
Now I'm using my Fritz!Box 6590 as a cable router again.

Greetings, Thorsten