@wasikhan 

Apologies to respond late.

Please help confirm some details, we will try to figure it out ASAP:
1. Ensure you are running the latest firmware on your MR6400 first.
2. When you mention the connection disconnects, are both the wired connection and wireless connection dropping when connected to the MR6400? Can you still see the wireless signal from the MR6400 on the wireless devices? Try to modify the wireless settings, such as wireless channel to 1/6/11, channel width to 20MHz, then reboot the router and reconnect to the network to confirm.
3. Who is the SIM card carrier? Are you able to get a stable internet if you put the SIM card on a phone?
4. Please confirm if the band is changed to a different one when it loses the connection, you may compare that when the internet is working fine and when you lose the connection. Try to manually select one by updating to the latest beta firmware you have posted here.

@FernandoC 

Hi, sorry to tell you that we can provide an English service for global users now. You are suggested to contact the local tech support to get some suggestions.

https://www.tp-link.com/support/contact-technical-support/

BTW, it is suggested to open your own post if you have a totally different issue with the original poster, as the issue you reported here is irrelevant with the original.

Good day.