You can choose what happens to your ad unit if Google has no targeted ads are available. By default, Google will try to collapse the ad space on your page or if that's not possible show a blank space. Alternatively, you can choose to display a solid color or another HTML page. This way, you can make sure that your advertising space is always being used effectively.

If no Google ads are available for your ad unit [support.google.com]

Google basically just gives the ad space back to you, so you can fill it as you like. Ads will be loaded in an iframe, so your other ad network will have to support that.

What do you guys recommend?

As robzilla suggested you consider using the backup ads. I don't recommend removing the AdSense code from your pages, unless you have given up completely. Removing the code will stop sending requests to AdSense and as a result your site will no longer be crawled. If in the future you decide to show ads again there will be a lag before AdSense can figure out what your sites is about and the best types of ads to show.

If you don't have another Ad Network to work with, you can create your own ad either promoting another site (this is what I do) or promoting some engaging content elsewhere on your site. If the coverage issue is related to the "brand-safety" update, promoting content on the effected site may seem pointless but what it does is send few visitors from each of your many pages to one or two pages. The few visitors are aggregated from few to many and all directed to few pages, so those pages will not be impacted. Obviously not all user will be clicking on those ads,but I imagine that the click through rate should be pretty good as the ad will be relevant and you are not asking the users to change website.