Wireless network (802.11ac) problems with OS X El Capitan and different region codes

Question

I use an TP-Link Archer C7 v2 as router (802.11ac/5 Ghz) and a Macbook Late 2013 with Mac OS X El Capitan (10.11.3).

Normal speed in OS X is around 800-900 MBit/s. After wake from sleep or restart i have drops to 7 MBit/s to maximum 400 MBit/s. So I investigated:

1. Reinstall Mac OS X
2. Set Router to factory defaults
3. Everything works with normal speed
4. Set region code in the router from United States to Germany (both times channel 48)
5. First it works but then speed drops to 7 MBit/s - 400 MBit/s

Sometimes restart/reconnect helps and speed goes to normal with German country code, but drops after wake from sleep for example.

Any ideas?