No, the line in stays on if the internet goes away. 

I would be guessing that your phone stopped playback of whatever source you were playing when the internet disappeared. I know my phone does not gracefully handle a switch from Wi-Fi connection to 5G when playing a source from outside, it requires me to 1) turn off Wi-Fi, then restart the source. 

Thanks for the answer. I thought i did do that. 

Establish line in in app

Go to Bluetooth 

Play over BT

Disconnect from wifi on phone before internet drops.

Ill double check.

You were right. If i disengage wifi on the phone after line in set up its maintains the connection. 

Your a bit buggered if you have no Internet though initially though. 

Anyway thanks again Bruce.

Pretty sure you need internet only to actually turn on the line in, making it functional. Once it’s ‘available’, it should recognize its being used whether there is an internet connection or not. 

@Adrian_16 .  I note that you say you have Internet on app. 

So do you mean that your phone is connected to your WiFi LAN and hence to the Internet? If not, what do you mean?

What precisely is the source of the music playing on your phone?

When you say that Internet is lost, do you mean that the router loses its connection to the Internet but maintains a WiFi LAN? Or do you mean that the WiFi LAN crashes?

In your line in settings in the Amp, what is the Autoplay Room set to?

I don't feel the scenario is anything like clear enough to work out what is happening. Maybe I'm being dim.

Since the internet is iffy i cant stream i.e. spotify connect so i was working around it by bluetoothing to line in adapter. So my phone was connected to mobile data ONLY, AFTER i had established the line in during a period of internet access.

Basically since there is no input select on the amp physically i.e button / toggle etc you have to be able to select line in before the internet drops; since the app is not functional without internet/+wifi access. 

So in the scenario of no internet (but possibly wifi) you cant use your line-in as unlike the hdmi it has no auto detect / auto play functionality.

No internet at all = no line-in at all. Once you have line-in established (i.e wifi+internet briefely comes back and app comes back) then you can maintain the line-in should internet (and possibly wifi) go off but you phone is on mobile data (4G) only (in my bluetooth scenario). 

Im assuming if your using cd player as line-in source, it maintains once established but even here you need internet+wifi to make the player use the line-in at first before you can play anything.