Hello!

First of all, I am sorry if this topic has been already covered, but I cannot find any related thread, so I would be really grateful for redirecting if that is the case. 
 

I am planning to use my future MR6400 only as an LTE internet source, with wifi disabled and to forward the connection using ethernet LAN port to WAN port in my Apple Time Capsule router and use it to broadcast wifi signal. The reason is that the Time Capsule supports wifi ac and the MR6400 only supports wifi N, which would slow down my wireless backups to the Time Capsule internal HDD, as my laptop also supports wifi ac. The LAN ports on MR6400 support only fast ethernet (100Mb/s) so I am aware that this will be my theoretical maximum internet speed using this method. 
 

My question is which settings apart from turning off the wifi radio on MR6400 should I change on both routers to achieve best internet speed and to avoid problems, as I guess that routers are set by default to be the main device receiving internet connection and managing network. (I read about double NAT problems but I am not sure what is is). I believe that the second router brand (Apple) does not matter as I presume settings which would need to be changed are common to all router brands. 

I will be really grateful for responses or for links if this topic has been covered. Thank you!