This is completely normal behavior for self-signed certificates.
NET::ERR_CERT_COMMON_NAME_INVALID is a message that is shown as the self-signed certificate is not verified by any certificate authority. Additional to this the certificate is created in regards to the servers hostname/IP address.
The reason you are seeing the notice after a couple of days is that your browsers cookies are no longer stashing the certificates requests. You can test this by accepting the certificate and then clearing your browsers history, upon opening the browser and navigating to your site, you will be greeted with this message.
One way to combat this is to purchase a domain and to set the IP to an A record on a DNS provider such as CloudFlare, once that is done you can cover your domain with an SSL from Let's Encrypt. However, if you have it running on your local network such as what the IP shows, you would need to use a service such as FreeDNS.Afraid to forward it to a publicly available domain.