@SLohia 

Do you want to attach Deco mesh to these two routers, or do you plan just to run these two routers MR600 and ask if they can form the mesh?

@Alexandre. I want one mesh network in the house, with the internet connectivity being provided by both MR600 routers to this single mesh network. I do not want to have two different mesh networks, one around each MR600. Is this possible... having more than one Master Router on the same mesh network?

@SLohia 

I still need you to clarify what mesh do you plan to use - routers configuration will depend on it.

MR600 supports OneMesh, which is range extenders you can buy to expand your network, they are seamlessly integrated with router and expand router WiFi network.

TP-Link also offers Deco mesh, which is technology that can be used with any router. Set of Deco units creates its own WiFi network, Main Deco is wired to router (such as MR600), it is recommended to turn router WiFi off to not conflict with Deco WiFi network. 

Finally, you may not be looking for adding another hardware and just want to have two MR600 WiFi routers providing one WiFi network and have WiFi devices connected to it.

Assuming I can help you to figure how to run two MR600 seamlessly, so that if any one 4G network is down your house still has uninterrupted Internet, and I believe I have an idea how to do that, will you eventually:

1) Add OneMesh WiFi extenders;

2) Add Deco mesh system;

3) Won't add anything or undecided?

The reason I am asking is because MR600 routers may have to be configured slightly differently depending how you answer this, and I'd rather recommend you single configuration that fits your plans.

@Alexandre. Thanks for the detailed response.

So, you have understood my objective. I would like to have two MR600 routers on the WiFi network so that if one 4G network is down, I can use the other. I am happy to buy one more hardware devices to achieve this. So please advise what options I have and how can I go about achieving the objective I just set out.

@SLohia 

I think I have an idea how to make it work, but you will have to test it because I do not have MR600 router and can only rely on documentation. In solution I propose I will be referring to the following TP-Link document: How to set up the 3G and 4G Backup on TP-Link LTE Gateway?

The principal part of that solution will be feature of MR600 to fallback between WAN and cellular, and I am going to use it even if you don't have wired Internet. You will see where it applies. 

If this feature is implemented by TP-Link correctly, my solution will work - and that is for you to test.

But, first, I would like to better define what you want to achieve: "How can I configure two single-SIM routers to behave together like one dual-SIM router?"

If you put it that way, you'll see what you ask is not relevant to mesh, even if you use that word, and definitely not to Deco mesh. You asked your question at the wrong forum. The correct TP-Link forum to ask your question would be 3G/4G Routers/MiFi

If you check that forum, you'll see they mention MR600 a lot, that tells you it is the right place.

Yet, you've got my attention, so, as promised, solution below. I suggest you also post same question the way I phrased it to the 3G/4G Routers/MiFi forum and see if folks there can offer something better. Feel free to share there what I propose, for review and feedback.

The first step is to wire two routers together, i.e., daisy chain them like that:

Connect Router2 LAN4/WAN port with Ethernet cable to one of Router1 LAN1 - LAN3 ports.

The next step is for you to decide which one of two SIMs you have will be your Primary SIM and which - Secondary SIM. Primary SIM is what should be used most of the time, so choose one that belongs to more reliable cellular network. If they both equally good (or bad), then choose one that is faster as Primary, and if they both provide about same Internet speed - it does not matter which is which.

I will refer to these SIMs in routers configuration.

How to configure routers.

Router 1

1) Use Primary SIM

2) Set Operation Mode as 3G/4G Router Mode

3) Turn WiFi off 

4) Routers should manage different IP ranges. Configure Router1, for example, to manage 192.168.100.* network. Then, configure your Router2 to manage 192.168.101.* network.

Router 2

1) Use Secondary SIM

2) Set Operation Mode as Wireless Router Mode

3) Enable 3G/4G Backup (Go to Advanced > Network > Internet)

4) Keep WiFi on

5) Routers should manage different IP ranges. If Router1 is configured to manage 192.168.100.* network,  configure Router2 (this router) to manage 192.168.101.* network.

----------------------------

How it will work. 

When Router 1 connected to 4G, all traffic from Home Network will pass through Router 2 to Router 1 to cellular network. When Router 1 loses connection to 4G, Router 2 should recognize that wired (through Router 1) Internet connection is lost, and fallback to its own, Secondary SIM.

If you find that having one router with WiFi, Router 2, is not enough to provide good WiFi coverage at your home, then you'll decide if you want to add OneMesh WiFi extenders or Deco mesh.

If adding OneMesh WiFi extenders, they should be linked to Router 2. If adding Deco mesh, Main Deco should be wired to Router 2 and WiFi on Router 2 should be turned off.

@Alexandre. Thanks so much. Yes, your solution appears to make logical sense and does appear to provide a good 4g backup to another 4g router.

However, unfortunately I think I did not frame my question correctly. And this is totally my fault. What I was hoping to achieve is that if one of the 4G networks offers really poor bandwidth (often I have observed that speeds drop to <1Mbps), then network traffic gets directed to the other 4G network. Sort of a load balancer.