There are options, depending on where you live. I found one on Amazon (https://www.amazon.com/Deal4GO-Wireless-Antenna-Replacement-X939878/dp/B07Y2N4QPZ) and it appears that someone had the same problem as you, so here are some ideas that were suggested: Source A Replacement WiFi Antenna? Hope this helps!

The best you can do is try a new battery or fix the old one if you can and know how to do so.

As a Custom Built, make sure you have connected the Start button to the right pins on the motherboard. You can find the correct ones with some help of the motherboard itself or a guide or tutorial, depending on the model.

Before doing anything, make sure your battery is fine, as a swollen battery is the most common reason for a popped screen.

First of all, check that your phone supports the kind of SIM you are using (which will probably not be the case). Then make sure that there are no problems on your network provider company's side. Try updating (if there are any available) and restarting the phone a couple of times, as well as clearing the partition cache of the system. If you have replaced the SIM slot and the problem still persists, having also checked and done the above, the problem will likely persist. As a last resort, try to enable the SIM, if it is recognized, by following these steps: Go to settings, then "Network and Internet", then "SIM card and mobile network", then "SIM card Information and Settings" menu, select the desired SIM card, then slide the "Activate SIM card" to the enabled it, and done! Hope this helps!

Try restarting the phone, and checking your phone's system health (you can do this by opening Settings>Battery and Device Care and performing the necessary actions for each category). This will also help you free up space with the help of the corresponding tab. Additionally you can also check for system updates by opening Settings>Software update>Download and install.