I would rather just give up. I had 10x Mr600 and MR400 and MR200 mix. All same issue on different ISPs or carriers. Younare wasting your time. Best suggestion is to take router back and get something that works. You can follow couple of similar threads on forum. There is no solution. Terrible products. Cannot believe that TP link would even put them on market still knowing that they dont work. I have decided to never use any TP link product again. They have ruined some of my business reputation and made me look incompetent in front of more than one client. So that is the best solution I can give you. I would suggest a Huawei B535 or B818 router. Have never had any issues with them at all. And some of them you can even select LTE band. A cost effective router with loads of bands and also very stable is the zyxel.LTE 3202. Proper stable router. 

@GT73 see my msge above. Sorry to be the bearer of bad news ..

@ne1gold 

First I thought my old Huawei b315 936 has the issue so I bought another new TPLink router then connected it to Archer MR600 LAN port. The new router still has internet problem after 3 days. Its so strange coz the other TP link EAP110 that was also connected tot the modem MR600 don't have problem at all. There were few threads about this issue the other one was more than a year ago. 12 pages of posts but no solution handed down by TPLink support. So shame doing business that can't solve issues.

@Kostek "This product DOES NOT WORK as LTE MODEM"

In the last 3-4 weeks running it as just a WiFi router, I've not seen it slow down, crash or need rebooting. This POS hasn't just got a terrible LTE modem (and astonishingly misbehaving antenna - running better WITHOUT THEM sometimes), simply using LTE cripples the router part of it as well.

Without LTE for the price I paid, this is a woefully under performing router. Really wishing I'd spent the same money on a good gaming router and used the ISP's Huawei LTE router as a modem, it was a reasonable LTE modem in a terrible router!