You need to provide more information. There is a support template here on the forum that requests additional information for a reason. We can’t help you if you don’t give us what’s needed to help you. 
- What version of Nextcloud Server? What version of the Android client?
- What, precisely, happens when you upload larger files?
- What appears in your Nextcloud log during these failed transactions?
- Are you using a reverse proxy and/or something like CloudFlare?