@JoshVDH 

I have the same problem. Started around 2-3 weeks ago.  I have the MR600.

@rebrooks 

What mobile network are you with?

Think I've come to the conclusion that it is a problem with smarty (Three).

They seem to be having a problem with their 4g and 3g networks all over the country. Lots of people complaining on Three's Twitter account. Something to do with the DNS routing.?

I've decided to change network providers, mainly after contacting smarty for support and how rude the customer service were basically telling me its my problem and blaming poor signal in my area (which isn't true)

New sim is yet to arrive (gone with Vodafone) so I haven't tested it in the mr6400. But considering the mr6400 works fine with my o2 sim from my mobile phone, I can assume it was the three network that was causing the drop outs. 

@JoshVDH 

Yes it's Smarty. Hmm.  Maybe there are two problems?  Three/Smarty *and* the router...

Remember that Three/Smarty is the only 4G provider not using CGNAT. Particularly relevant if you want to play online games. 

@dgr1 not sure what CGNAT is or what implicationthat has, sorry my knowledge on network systems isn't the best. 

I have been using smarty sim in the mr6400 for a past 6 or 7 months though without any problems. 

@JoshVDH No problem. Carrier grade network address translation (CGNAT) is used by EE, O2 and Vodafone on their 4G services. It is linked to the lack of IPv4 addresses, and CGNAT allows the providers to share these amongst users. Three do not use CGNAT (at present!) and Smarty use Three's network.

On a games consoles, if you run a network test you will see that NAT type is reported. I have a PS4 (reports NAT type 2) and a Switch (reports NAT type B) which are both fine for online games. To be clear, if you don't play multiplayer online, none of this is a problem.

Whichever network you try, I suggest you run a connection test to see your NAT type.

I get better speeds from EE and Vodafone where I live, but am with Three for this very reason. My kids would not be happy if they couldn't play Minecraft with their friends!

And it appears that Smarty also fail to give you a unique IP, so Three is your only option in this regard. 

@dgr1 yep, got my Vodafone Sim yesterday and now understand what your saying! Loaded up a PC game and found my NAT type was 'strict' (nat type 3) slightly annoying but I've found using a VPN is a way around it. Using it give me a type 2 NAT (moderate) Currently using a 7 day free trial to see how it goes. Not entirely sure I want to pay an additional £7 a month for a VPN though. 

@JoshVDH Yes, it is very annoying and something you only find out about once you have a SIM card and find you get good speeds..... and then realise gaming doesn't work! You will be ok if the VPN doesn't impact your speed too much. And some VPN services have quotas, so watch out for that too.