Why is my wifi connectivity so poor?

When you say antennas, do you mean the antenna in the iPhone (http://www.etradesupply.com/oem-apple-ip...)?

This is perhaps a dumb question but is it properly connected, especially the connection underneath? They don't always mate very well but can look okay. The other thing I would verify is wether everything in that area is well connected.

See this image: iPhone SE Logic Board Replacement

All of these elements are very important and I often see phones repaired by owners that had missing elements, screw etc. These can be very important as they complete grounding circuits and can have an impact on functionality.

Finally, you may want to inspect the upper portion of the backside of the logic board. That is where the WLAN/BT chip resides. Use a magnifying glass or better yet, a 10X loupe and inspect for any anomalies.