First, this is my first real foray into the network world (hanging on by my fingernails).

Provider is Xfinity.  Recently I stumbled across Pi-Hole. So I equipted myself with a Raspberry Pi 4, keyboard, mouse etc. "Took" a few lessons from a YT channel on "Networking for Dummies". Followed a YT channel on "how to install" Pi-Hole. 

Soon found out the supplied Xfinity modem/router (for home use) does not provide a path to define a static IP for the Pi-hole, which is required. So after a little research I set the Xfinity into "Bridge" mode, purchased a TP-Link AX21, set it up and defined a static primary DSN as the IP address of the Pi-Hole.  Crossed my fingers and fired it all up. Much to my delight (and a bit of surpise), it all worked! For a while, that is. Everything worked perfectly for a coupla days(I Love not seeing those ROKU ads), then one morning the "internet" led on the front of the AX21 was red. Now what was confusing is I had internet connectivy but all my local stuff (iphone apps for the garage door opener, my yard irrigation app and others) would not respond. I fooled around a bit (don't actually remember the specific sequence of events) but you know the drill, power off, disconnect stuff, recoconnect, power on, wait a while. Eventually I glanced up at the router and the internet led was green. Checked my apps... all working.

Fast forward a day or 2. This happened this morning the second time. Couldn't get that darn internet light to go green. Finally set up a Chat with TP-Link. Like I sorta expected, basically they wanted me to do a factory reset on their router. Which I translated into (they don't have a clue what is actually wrong). So I politely thanked him for his effort, and terminated the chat. 

So the short version of my question is... if that led is the 'internet' indicator, why, when it goes red, do I still have internet but cannot launch my local stuff from my iphone. And of course, why does it go red in the first place?