How do I reset the receiver to search for the new WAP?

It's standard 5 GHz.  All of our U-verse wireless receivers are connected to our home WiFi network.  The AT&T WAP is powered-off and sitting in a storage box.  We have a perfectly good WiFi network and don't need yet another access point hanging around occupying extra wireless bandwidth, physical space and power.

I'm surprised the tech recommended you to use your own WAP.  There are some requirements that must be met...  Your WiFi router/access point needs to perform IGMP snooping.  Additionally it must be able to convert multicast to unicast on the wireless side.  If your network equipment specifications calls out IGMP snooping, IPTV or IP Multicast support, then you're probably fine.  Keep in mind that the U-verse wireless receivers have 5G radios only so they will only associate with your 5G SSID.  WPS is also required for authentication.

U-verse IPTV uses IP multicast.  Each TV channel is a multicast stream.  Without IGMP snooping, this multicast traffic will be sent to every port of your router, and to all downstream switches, including the WiFi network.  Ideally, traffic from each stream is supposed to go only to the receiver that requested it.  This means a lot of extra and unnecessary traffic will appear on all your devices.  Each video stream sends at about 6 Mb/s of traffic so a full six streams of HD video will cause an extra 36 Mb/s of useless traffic everywhere.  If your router/AP does not convert multicast to unicast, a similar situation occurs on the wireless side, only worse.  Multicast traffic is sent (just once) to every wireless client but using the basic rate which is generally the slowest wireless rate (that all clients can support) and it may be as low as 1 Mb/s.  This will certainly kill your wireless network.  IGMP snooping combined with multicast-to-unicast conversion will send the video traffic to just the wireless client (receiver) that needs it and since the traffic is converted to unicast, it is sent using the highest available rate, just like normal traffic.

To associate your wireless receivers to your own WiFi network:

1.  Power-off the AT&T WAP  (if you are successful, you won't be needing it again).

2.  Power-on the wireless receiver.  It will not find a network so follow the on-screen instructions to establish a new connection. Namely...

3.  Press the WPS button on your router/access point.

4.  Press the OK button on your wireless receiver.

If all goes well, within a few minutes, the receiver will find your WiFi network and associate with it.

If your equipment supports multiple SSIDs, I would highly recommend configuring additional separate 5G SSID for each wireless receiver.  This has nothing to do with performance nor separation.  It is about being able to disable the wireless access to each wireless receiver individually should you ever want to convert to a wired connection.  If you don't disable the wireless side of a receiver when connecting it to your network using its RJ45 port, a loop will form and bring down your entire network.