Problem using Time Capsule as Router

Question

I just purchased an Airport Time.Capsule.802.11AC 2TB router for my home, which for the last year worked happily with a an inexpensive router.  My home system is reasonably complex, with a guest house with an Airport Express used as an wireless access point, and and Airport used in another part of the house.  Both the Express and Airport extend the network (and have done so in the past).
Since installing the Time Capsule 2 days ago, I have had no end of problems, first with getting devices to connect wirelessly;  I seem to have (generally) solved this problem by deleting the old wireless details and entering the new wireless access details (even though the access point name and password remains the same).
Still, problems plague me.   The Airport seems to intermittently report that it cannot extend the network;   internet access speeds are sometimes slow or non existent (last night, download speed was 2MB/S, upload speeds noon existent).   I suspect this must be because of some conflict within my system: I use a cable modem, and right now all wireless clients gain access via DHCP.    
I've attached my 'settings' for the router.
Is there a setting which is obviously out of place, or some other idea as to how to debug?  I wonder if the router is using a new wireless protocol what older iPads/laptops/etc cannot speak to as quickly, but dismissed that as somewhat illogical.

Answer 1

How can I determine if it is a firmware fault?
Because you test precisely for where the fault is..
To do that you measure local lan speeds, connectivity and reliability using ethernet and wireless.
And then do the same with WAN.. you will discover where the problem exists.. if it is wan speed or drop out then you can be very sure it is WAN bugs.
This is as I stated an issue with the AC model.. and all the firmware seem to be affected but you should be on 7.7.3 as the earlier ones are not secure.
Why would Apple ever release such a device with a known fault? 
The following is pure speculation .. and might get deleted.. !!
Why did apple release a bad update for iphone 6?? Lack of testing.
http://bgr.com/2014/09/25/iphone-6-and-iphone-6-plus-ios-8-0-2-release/
Why do they produce phones that bend? Lack of testing.
http://www.macrumors.com/2014/09/25/apple-responds-to-bending-issues/
This is more typical apple response .. ie in normal usage they are fine. ie it is end users that are at fault. Should not wear tight pants.
In networking there are huge numbers of products in the marketplace. To get reasonable products to market you need to test vigorously and update continually until you find all the bugs.. Since Apple released the AC model there has been two firmware updates.. one on the day of release.. and one recently with the ssh bug.. (one other was withdrawn after a few days.. as typical it caused more problems than it fixed.. which is lack of testing). For a new leading edge product this is totally inadequate. But I am sure it worked fine on the two different cable modems they tested it on.. and 3 different dsl modems and 2 different fibre setups.. unfortunately there are thousands upon thousands of modems and routers out there. A large beta program is required.. but Apple could not do that as it would be releasing hardware without marketing fanfare.
I personally have these issues with the Airport Extreme and Time Capsule on my ADSL.. it simply does not work reliably. No other router I have tested fails.  I have tested lots.. but the Apple cannot maintain a pppoe connection to ISP equipment. All other routers can. For some it works perfectly.. for some it doesn't work at all.. for me it drops out at 2min intervals. It is simply unsuited to the task. Find another way to use it .. or return it. Clearly they don't get enough returns to believe the issue is that big.
In the forum we deal with it every day.. eg today.
ISP Speed issue after installing TC
Just do a search for SB6121 or SB6141 which seem particularly prone to the problem.