By default slingbox M1 use port 5301 within your routers firewall , try the below link from a pc connected to the same network and check the port 5301 if its open
if not, you can contact your ISP and assign the port 5301 to the slingbox.
I have a Galaxy Note 4 running Android 6.0.1 and the slingplayer app version 2.0.4. I have a Chromecast model NC2-6A5 named chromecastdc with firmware version 1.20.66189 attached to a Visio 32" flat screen with 1080p resolution...I have a slingbox m1, firmware version 1.2.92, hardware version 1.0.1...all of these components are on the same wifi network, which is run by a TP-Link Archer C8 router...and when I run speedtest from my phone over wifi, my upload speed is 26.85 mbps and my download speed is 6.57 mbsp to the internet...of course my network is at least this fast inside my house not going to the internet..When I try to Chromecast my slingplayer, the screen on my phone says "switching to Chromecastdc" and the television screen says Slingplayer Build 1.0.42. There is a blue semi-circle going around on my phone,...I have waited for up to 5 minutes, but the phone and the tv never connect...the screens just remain as I have described.
Any ideas why the picture that the slingbox sends to the phone never gets to the tv?