No service + Bad WiFi reception

Hi all!

I’ve recently bought an iPhone 6s 64GB secondhand, wich would have faulty GPS. No problem, I was going to use it as a second phone just to receive calls and play some music.

However, after inserting my new SIM card I only get: “No Service” The iPhone detects the SIM card just fine, and all software troubleshooting (Reboot, flightmode on/off, complete reset, etc) didn’t work. SIM is brand new and works great in other phones.

WiFi does work, but it’s reception is very limited (Only my own router when I’m really close to it).

After searching online, I don’t think my Baseband chip is at fault; *#06# displays my IMEI just fine. People recommended that I swap out the dock connector board (with antenna). And I did.

Unfortunatly that didn’t improve things. Things stayed exactly the same.

Is it wise to replace the other antenna (that is placed by the seaker) as well? Or my GPS module? Is there anything I can do to rule something out?

ps. The iPhone was never opened, nor where there any liquid indicators tripped

pps. I’m sorry for any errors, English isn’t my first language

Many thanks!