@AlexLitu 

Honestly it sounds like the app on the TV,  If HBO-Go works on your laptop and you are connected to the router, LAN or WiFi traffic would not be an issue.  I have seen app issues on smart tvs in the past.  Usually removing the App and reinstalling it, or updating if an update is available are the best bet.  if that does not work you would want to reach out to LG to see if there is a bug they need to address in their software.  

@AlexLitu 

That is a common exceuse for ISP to blame the rotuer.  They have no real training on the products nor any real test that can prefrom that can determine such a judgement.  They are only guessing.  Now based on what you have told us here the issue is the App on the TV is self.  We can with confidence determine this because of the following factors. 

HBO-Go works on your Laptop, All of the other apps on the TV work.  These two items show that not only is the router sending and recieving data from these streaming providers, we are specifically sending and recieving from HBO-Go on a seperate and different platform.  Our recomendation would be to talk with the TV manufacture and let them know the App is having issues.  

@AlexLitu 

The frequency is something we can help with.  What they are referring to the the wireless channel.  We see issues simmilar with Apple products occasionally.  Apply likes to operate on the upper channels of the 5 GHz  specturm.  This may be a similar situation.  What you would do is log into the router and change the wireless channel settings for the 5 GHz.  see the screenshot below for the order of menu and clicks you need to go.   Change the Channel Width to 40 MHz and the Channel to one of the 4 higher number channels then click save.  From there i would recommend a reboot of the router just for good mesure, then see if its working.  

Though typically you would have issue with the other apps but I've seen stranger things so I'd say give it a shot.