In what way is your pc not doing the job?
If gaming performance is not good, then why not?
Your graphics card is as good as it gets and is more than enough at 1080P and 1440P.
Your cpu is very good, but some games depend heavily on the performance of the single master thread.
A i7-4790 runs stock at 3.6 with a turbo of 4.0
You could replace it with a i7-4790K which runs at 4.0/4.4, only a 10% boost. But selling the 4790 to pay for the upgrade might not be very costly.
Here is a backhanded way to see if cpu speed is an issue:
In windows power management, limit the maximum cpu% to something like 90%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 10% improvement in core speed might do.
If you do not have a SSD, that would be my first recommendation. A SSD does more than boot faster and load games faster. It also does game checkpoints faster and loads graphics textures faster. Samsung has a nice free utility to do a ssd migration.
12gb is fine. Likely, 8gb is running dual channel and the odd 4k is running single channel. Not the best, but much better than 8gb alone. No need to change the ram.
Can't help on the internet issues.
Research them for your provider. There might be some settings that are applicable.