If you set the portfolio template doesn't work as a homepage and it works, but as a regular page it doesn't.
Please make sure that you don't set the portfolio page's slug to portfolio. It can be anything else, like: projects, work, our-portfolio, etc..
If the theme doesn't work right. You might need to configure the permalinks in order for everything to run smooth:
Go inside WordPress Settings > Permalinks and choose a "Custom Structure" for them, then use this setting in order to make it work: /%category%/%postname%/