Flash content is getting blocked in the latest versions of browsers. To enable it in Google Chrome,
first open your browser and type chrome://settings/content/flash in the address bar or go there from
menu "Settings / Privacy and security / Site settings / Flash". On a flash setting page, set toggle
to Ask first (recommended). Now, with Flash enabled, whenever you visit a webpage with Flash content,
you’ll need to click on the Flash player for it to start.
Days 2 Die - The Other Side
If you want to play this game offline and without any browsers after December 2020, then download .swf game file and use any Flash Player that runs .swf files.
For Windows users, I recommend ARFlashPlayer (it supports fullscreen option) or official Adobe Flash Player. Both programs are portable.
Another way to play flash games is to install SuperNova SWF Extension (it works in Chrome only).