Question about LC-Power LC-M34-UWQHD-100-C

I bought the monitor new a few days ago and tried to connect my Macbook Air (2015, early) with an adapter (Mini DP to HDMI) and an HDMI cable. However, only a too small scaling format was recognised, which is why I could not use the full width. After research, I realised that it could be the mini adapter and ordered a new cable from Lindy (mini DP to DP) so that an adapter was no longer needed. As soon as I received the cable and connected it, the 3440x1440 scaling was indeed suddenly redisplayed and I was already rejoicing. This was proof that it was due to the adapter or HDMI cable. Unfortunately, I noticed afterwards that the picture appeared wider, but a small border remained white on the right and left. Now I am wondering what else I can do to finally be able to use the full area with the correct scaling?