UNABLE TO CHECK FOR UPDATE

You might be unable to update your iPhone for one of these reasons. 

• Not enough available storage space on your device 
• Downloading the update takes a long time 
• Unable to reach the update server 
• The update doesn't complete 

Hope these Apple Support links can solve your issue. 

• If you can't update or restore your iPhone, iPad, or iPod touch 
• Get help with over-the-air iOS updates 

Hope it helps! 

There are several things that can happen here:

1. You're already on the latest version of iOS. If you want to install beta, make sure that you have beta profile installed.
2. There's some iCloud glitch. Sign out/in to iCloud and check if you can update iOS.
3. As mentioned in other post, there can be a lack of local storage. Make sure you have 5-8 GB available.

Also, don't forget that you can always update with iTunes or Finder. If you have an old macOS version, just use Windows instead (it can be installed on a VM or bootcamp if needed).

Unable to check update and there is no signal