My wifes phone keeps reaching out a malicious site (udmserve.net), and it is being blocked by my router's Trend Micro network scanner. I ran Trend Micro's virus scan, as well as Clean Masters, but there are no results on her phone. Any idea of how to fix this? I enabled real-time protection from Trend Micro as well. It's a Samsung Galaxy Halo, which is a low-end Galaxy.
It sounds like the phone is infected by some kind of malware. You could do a factory reset to try to get rid of it. Doing a factory reset will remove all data (photos, sms, files, apps etc)! Don't forget to back up your data if you want to keep it.
After the factory reset, only install trusted apps (use your instinct / google) or you might end up getting the malware again. Not everything on google play store is safe.
You could also try to identify the malware and delete it if you think you are capable. You could try to look for apps installed on the device that have the internet permission (since the malicious app is trying to reach that website, it must have the permission) and use google to see if the app is known malware.