Let's now jump to conclusion so quickly gents, after all we have only one environment and 30 different devices that is experiencing some issues. Most of the testing information for this model, come from running the router mode, which is the primary function of this home router, and the results have been great. AP mode may or may not have some problems, as found in this scenario, but it should not be the reason to put a label on the product.
Yes, calling the support line will get the attention of D-Link's technical services department and they may collect information and escalate to their engineering team, so there are benefits of calling them, but that usually takes some time to sort the information and look for the solution once the issue is found. Depending on how soon you need this fix and how much time you are willing to wait for it, this may not be the best solution. If you are willing to stick around, I'm wondering if you would be willing to maybe run some test on BETA or RC firmwares, or if this may not be possible since this is a production network and all... Let us know what you think.
Like in the case of FurryNutz, I run about 37 devices in my home network, but only 27 of them are connected using wireless to this router and I have no incidents to report so far with Smart Connect function. Maybe because most of them are 802.11ac with the exception being a printer and two old laptops using 802.11n built-in adapters, I'll try to see if I can replicate something in my network.
This model with three Wi-Fi radios (One 2.4 GHz and Two 5 GHz) handling 30 clients should not be something to consider a problem as long as there is combination of 2.4 and 5.0 GHz clients. In my personal opinion, with each radio supporting around 15 clients (theoretically speaking), this router would support up to 45 clients connection, again with diversity in the two bands.
Give Tech Support a try (ask to be escalated to a L3 technical agent) and let us know.