"Ethernet" doesn't have a valid IP configuration

Problem: intermittent no internet connectivity
Running network troubleshooter gives me the error message: "Ethernet" doesn't have a valid IP configuration

My system
Toshiba satellite laptop S75 hardwire to a personal modem arris sb6141, my ISP is Comcast xfinity. No router. I have high speed internet.

I have had the same setup for at least 2 years without this problem. All of the sudden about a month ago, I started getting no internet connection intermittently
Without any updates or changes to my system. The issue comes and goes, meaning sometimes there is no internet in the morning, I will come home from work at night and internet will be working.

I have talked to Comcast reps over the phone many times and they tell me it looks good on their end and that they can see my modem, they did a reset of my modem but it didn't help.
Comcast has sent 3 different techs at my place and they both said the wiring has no issue. I had the third comcast tech remove a splitter outside my apartment, but it made no difference for my connection.
I have replaced the Ethernet cable, but there is no improvement.
When it first started I had windows 10 pro original version on my laptop, I updated windows 10 to build 1803 but it didn't help.
Then I updated windows 10 to build 1809 and internet started working again for a week with no issue at all.
Then the intermittent no connectivity issue came back.
The lights on the modem look good. There is no yellow exclamation point in the device manager. The IP configuration is set to obtain an address automatically. I have done a network reset. I need your help, please.
I can get wi-fi to work on same laptop (open xfinity wi-fi from neighbor).
I installed the latest drivers, the network driver version is now 10.31.828.2018 and I did a network reset, but it didn't help.

Welcome to the forum. It's rare to just use a modem the main reason being it makes you PC live on the internet were anything can connect directly to it unlike a router which stops any direct access as your PC is hidden. When it doesn't work does the modem lights for internet show it's still connected? Open a cmd prompt type ipconfig /all note IP address and the lease time date. If it's a short lease it could give problems when it drops do same cmd and see if it's lost its IP. If it's lost its IP type from cmd prompt ipconfig /renew see if it brings it back. Does it happen after its left idle?

Samuria said:

Welcome to the forum. It's rare to just use a modem the main reason being it makes you PC live on the internet were anything can connect directly to it unlike a router which stops any direct access as your PC is hidden. When it doesn't work does the modem lights for internet show it's still connected? Open a cmd prompt type ipconfig /all note IP address and the lease time date. If it's a short lease it could give problems when it drops do same cmd and see if it's lost its IP. If it's lost its IP type from cmd prompt ipconfig /renew see if it brings it back. Does it happen after its left idle?

Yes when internet doesn't work the modem lights for internet still show. Ethernet has been down since last night, so the current Ipv4 address is 169.254.xx.xxx (where xx are numbers I hide).
I am not sure what a short lease is, I noticed the lease is in general 3~4 days when ethernet works, it also shows a real Ip address not the one it currently shows.
I loose internet connection even when I am browsing sometimes.
From cmd prompt as admin, typing ipconfig /renew just says "Windows IP Configuration
An error occurred while renewing interface Ethernet : unable to contact your DHCP server. Request has timed out.
No operation can be performed on Local Area Connection* 11 while it has its media disconnected."

It's either a fault on DHCP on the modem or it could be a faulty cable. If the lease is in days it should only happen at the end of the lease. What are the settings on the network card if it's set to full duplex that can give problems. Do you always get the same IP if you do you could give the card that as a fixed IP. For safteys sake you should get a router every hacker in the world could connect to your PC as it's live on the net not protected by nat behind a router

Thank you all for your responses. I bought and set up a router and both wi-fi and ethernet (from my laptop to the router) seem to be working so far.