Bad signal and low mobile data speed, what to replace?

Hello,

I've been having unusually bad signal or good signal and bad mobile data speed for a couple of months now and I'm not exactly sure why; of course the phone is almost four years old so it’s had its shares of falls and I've replaced the battery myself a couple of times (successfully) but the problem started a few months ago. In the meantime I cleaned up my phone with a hard reset and configured it as new, so it shouldn't be a firmware issue.

I tried using a friend’s SIM card today, a provider with better coverage than mine, thinking it could be that even if I’ve never had any signal or data speed problems with my own - but noticed no changes. It works perfectly well on their phone, but the connectivity it was barely functioning on mine.

Some days it looks as if everything is fine and working, others I can't seem to start phone calls or visit web pages even if the signal bars are good (usually 3/4).

I have no problem replacing things on my own since I’ve had experience in doing it, but I just need to be sure on what to replace. Could a faulty antenna be the problem? If so, should I replace the Wi-Fi/Bluetooth antenna too or just the “top left” antenna?

Thank you for any info!