It might help if you provide more details on how you originally rooted your Galaxy Light and how you un-rooted it. (Presuming you used the 'Full unroot' option in SuperSU's Settings menu???)
You should try wiping the system cache partition any time some change is done on the base operating system. Boot up into your Recovery Mode and select the 'wipe cache partition' option.
http://www.hardreset.info/devices/samsung/samsung-t399-galaxy-light/recovery-mode/
If that doesn't make any improvement, you could try booting up into Safe Mode. While running in Safe Mode your Galaxy Light is just running a clean Android OS without any third-party apps/services getting autoloaded into the background. It your phone is working correctly in Safe Mode that's an indicator some app you've installed is the problem so then it becomes a matter where you need to determine just which one.
https://support.t-mobile.com/docs/DOC-32681
But this is more of a long shot, your problem doesn't sound relative to this, but it's worth trying.
A final suggestion is to just restore Android on your phone. You can try using Samsung's Smart Switch application. You need to install it on a computer, connect your phone, and run it.
https://www.samsung.com/us/smart-switch/
I'm a little doubtful in this particular situation it will fix your problem but at the very least it will install the necessary Samsung driver onto your computer.
If Smart Switch doesn't fix your problem, you might want to just manually flash a stock ROM,. You can safely download a stock ROM from here:
https://updato.com/firmware-archive-select-model?q=GALAXY+Light&rpp=15&order=date&dir=desc&exact=1
Be sure to pick the appropriate ROM that exactly matches your phone model -- do you have a SGH-T399 or a SGH-T399N? Anyway, on the actual download page there are instructions on what to do to flash the ROM. You'll need to use the Odin utility too. Follow those directions carefully but one item I'll disagree with is the version of Odin that's listed. Try using an older version, ver. 3.09 instead. Odin is not very backwards compatible and you're working with dated hardware and a dated ROM. Download from here:
http://www.droidviews.com/download-odin-tool-for-samsung-galaxy-devices-all-versions/
Also, go into your Windows Task Manager and be sure the Smart Switch utility isn't running in the background. If it is, just kill it so there isn't any conflict with running Odin.
Oh, and if you don't have access to a Windows PC there's also a similar utility, Heimdall, that's available for Linux, Mac, and Windows. You do need to make allowances to those same instructions accordingly but Heimdall is functionally doing the same functionality as Odin.
https://glassechidna.com.au/heimdall/