I just set up my new Deco S4 mesh system, and realized I may have made a mistake during setup.

My goal was to increase connectivity through my apartment, but not necessarily create a new network.  I currently have an AT&T BGW320 modem+router unit.  I set up the Deco S4 units, and created a new network (didn't think to try using the same name as the existing network from my AT&T BGW320).  After setting it up, I saw that there is an Access Point mode, which to my understanding would just enhance the existing network connectivity throughout my apartment.

Currently I have Network A (provided by my AT&T modem+router unit) and Network B (provided by Deco S4).  I'm wondering if it is possible to just set my Deco S4 to Access Point mode and rename Network B to be the same as Network A, which would leave me with one network: Network A (provided by AT&T modem+router and "extended" by Deco S4)

EDIT 1: I've gone ahead and attempted this and it appeared to work; however, my PC over wifi to the BGW320 was getting 500+ download (I have a nice wireless adapter), and now I'm only getting around 190 download.  I noticed the same speed when I ran some tests connecting to Network B before changing the SSID.  Any ideas why this might be happening?

EDIT 2: Based on what I've said in EDIT 1, would I encounter any issues by just running my Deco S4 in router mode with a different SSID (back to Network A and Network B) and turning on IP Passthrough on my BGW320?  That way I could still preserve my speed on my PC, but still have the increased connectivity for other devices on Network B.  Essentially, only my PC and close-proximity devices would be on Network A and all other devices (the majority of them) would be on Network B.