Are you using the stock built in apps?
Have any of them gotten updated since before you had the trouble?
Also, this is a dated device, and providers are quickly moving to LTE service.
So if this device is not compliant, be ready to get a new device quickly.
In the mean time, have you tried third party apps?
https://www.simplemobiletools.com/
Simple Mobile Tools has a great collection of apps that replace the stock (usually junk) apps that come with a phone.
(I replace all of the stock apps as soon as possible on all devices, and Simple Mobile Tools is my goto for this.)
As far as the other issues with attachments, how much empty storage is empty on the device?
A good rule of thumb is to always leave the same amount of open space as there is RAM on the device.
Generally, this comes out to about 70% that you can use.
(Not counting any SD card.)
This gives apps room to grow as you use them (they shink back a bit after a time) and for them to work properly.
The idea that clearing some data helps for a short time may be a clue that the device is too cluttered in general.