Samsung smart TV only shows message "weak or no signal" after hooking up new DVR -

Did you note when you disconnected the old DVR how it was connected to the TV?  Did you connect it back the exact same way?  If you're not sure, can you describe the ports you're connected to on each end?

Have you checked the TV input settings to make sure it corresponds to the TV input you're connected to?  

While troubleshooting your failed DVR, you may have punched a channel button on the TV and now it's trying to get OTA TV without any antenna.

Thanks for responding.  when I go to Menu / Broadcasting I chose "Cable" as my preferred method of receiving  digital broadcast (the other choice being "antenna") - that seems right.  would the problem have anything to do with Auto Program / searching and storing channels?  it seems to show that i don't have any stored?

@rebecca9559 ,

The "Broadcasting" setting on the TV controls the channel/frequency assignments that the TV will expect to arrive at the coax input.  In short, its switches the channel-to-frequency mapping between the VHF/UHF channels of OTA (i.e. Antenna) and the CATV (cable) channels as described on this site.  However, AT&T U-verse IPTV delivers signal to your TV Receiver using UDP/IP over Ethernet or HPNA and has nothing to do with that chart.  If you use a coax cable between your TV Receiver and your TV, only then would you have anything to do with this frequency chart, and then you'd be using either channel 3 or 4, which are the same for CATV or OTA, so your TV "Broadcasting" setting wouldn't matter.

However, using a coax connection between the Receiver and the TV is a really bad idea, because it means all content would be down-scaled at the receiver to 480i to send to the TV.  Hopefully, you're using an HDMI cable or a set of component cables between the Receiver and TV.  That means the Receiver can send the TV the entire picture at 720p or 1080i.  It also means that you wouldn't be using the TV's Channel system to view your U-verse TV programs (thus the setting you describe still matters naught), but instead would need to select the proper input for the cable type you're using. 

The receiver is responsible for selecting the correct channel and sends it to the cable attached to your TV input.  So the TV won't have any channels stored.  The TV Receiver would control all of that.

So, what type of cabling are you using, and do you have the TV set to use the appropriate input?  (Usually controlled by an INPUT button on the TV's remote, or sometimes individual buttons such as HDMI, COMP.)

Thanks.  pretty sure we are using coax, same cable we've always had.  I don't have an INPUT button but there is a SOURCE button where i choose TV (that's the default, for regular TV watching) and HDMI (takes it to ROKU)

Okay, Source button instead of input.  That works.

So, if you're using a coax cable from the Receiver to the Gateway, try tuning to channel 3 or 4 using your TV's remotes channel buttons.  However, unless your TV only has the one HDMI input, you should look at connecting the TV Receiver via HDMI.  

In reading the OP, I get the sense they’re trying to treat the Uverse box like an old style cable box with the new TV.   
Use the HDMI setup as recommended .  If you don’t have an HDMI cable handy, go get one.  You should see a much better and sharper picture.   The Uverse box does have a setting for picture format, and typically 1080i is used and the TV should acknowledge that setting as well.  

turns out that the HDMI cable is the fix.  AT&T failed to include one in the box or note anywhere in the instructions or in my phone calls to their tech line - 

Thanks very much for your time and knowledge.

Yeah...I think I got lucky one time and a technician happened to have an HD cable with him, but it was so long ago, I can't remember.

I used to swear by NewEgg.com for all my cables and wires. They used to be incredibly affordable. Now, it seems like Amazon is equally competitive. Don't go to a "box store" like Best Buy, as you'll probably end up overpaying, or at least that was the case the last time I checked.