@Sebvienna  Here is the response I received from TP-Link UK Limited on my Amazon review regarding the issue.

"Dear Customer,

Thank you for the review and feedback!

We'll be more than happy to address the concerns you have about your TP-Link device.

We are sorry to say that Hardware configuration for the MR600 to support carrier aggregation bands (CA bands) is based on the hardware chipset used which supports most comman UK ISPs like EE, O2 etc. In UK itself, there are thousands of different carriers which makes it difficult to integrate all the CA bands. Either way we have made a note of the feedback provided by yourself and we will work on the future firmware updates to support these bands.

Should you require any assistance with one of our products in the future, please contact onlinesupport.uk@tp-link.com and one of our support agents will follow up with you directly.

We certainly hope this experience will not discourage you from checking out more of our great networking products in the future.

Best Regards,
TP-Link UK Limited"

@enRiched92 

Interesting response.. Still a bit vague, they talk about hardware limitations, then say they are going to possibly add 1+3 CA via firmware update.. 

If this is possible I would love to see this happen very soon, or both my mr600 routers will be getting returned or sold off

@Kevin_Z 

Hi, what are the others carrier aggregation band supported by Mr600 beside 3+40 and 3+7? 

I am not able to get any information on the supported carrier aggregation band over Internet. Did it support band 5+40?

I'm on Three and it works fine for me and is using bands 3 and 20 where I am.

I've just received my TP Link Archer MR600 from eBuyer today. I've popped in my SMARTY (Three) SIM and now notice that the speeds are not what I expected, as my iPhone and other devices get faster speeds on pure 4G. Having checked the band the router is connecting to, it's only connecting to band 1 - and now I've seen this thread, despite the router being promoted as being able to support it.

TP Link; I've always bought your products. I have a TP Link Kasa Smart Light Bulb, I've had TP Link routers in the past. But this experience has really put me off the brand. You need to urgently look at this and release a software update ASAP.

@yaughan 

Hey everyone! Check this post: https://community.tp-link.com/en/home/forum/topic/193174?page=3#23

I got in touch with technical support and the engineers, and they provided me with a beta firmware that enables you to manually select the bands.

@yaughan  Hi , I just bought one and had been reading this stuff on band agregation while waiting for delivery and thought - oh no I'm on Three!!

However straight out the box it connected to 4G+ , with router placed in my window. Tweeking the antena I got 10-11 Mbs and I live in the wilderness about 5 miles from nearest masts.

no phone reception here except upstairs in a window and no 4G only H+ in a few locations outside. 

However when I moved it to my upstairs attic space expecting better reception but I only got 1 or 2 bar icons and no 4G+ icon and only one band 1 connection

moving it back to original location - no change

reset to factory defaults - no change - useless!

Contacted support chat and they advised hardware reset with the paper clip

did that, and hey presto 4 G+ bands 20+3 again

So not sure what this reset does - it certainly doesn't reset any of the user settings applied - ie network connection and admin password, IP address etc all remain as set up prior to reset.

Possibly all you need to do is try this rather than risking a FLASH of beta firmaware as that can go very wrong so easily!

Hope this helps some of you who have been grinding your teeth over the problems with Three sims in this router.

@pMR600 I am also using Airtel(India), but  4G+ icon light in the router is not ON. Whats needs to be done, how to check / fix the issue ?