Using the recovery exe with wine worked fine for me, btw. Just running it was enough. (and changing the IP address before of course so you can reach 192.168.178.1)
soooo..
excuse my late reply, I had to work but could reinstall FritzOS just now via the method proposed:
Wine and the exe.
This was very!!! useful because I couldn't manage to install the old FritzOs via CLI.
Thanks @2gltx
Like expected, with stock firmware I get full DSL speed:
verbunden, 63,7 Mbit/s12,7 Mbit/s
So any Idea what now? @janh
Thank you very much for all the support..
Hi! 2gltx here, which was a bugmenot.com account that just got banned. Bugged me enough to make my own account now 
Maybe it helps you if I outline how I setup my own FritzBox-3370 after a fresh snapshot install:
First I transfer the VDSL blob using scp:
scp /PATH/TO/vr9-B-dsl.bin root@192.168.1.1:/lib/firmware/vr9-B-dsl.bin
Then I ssh into OpenWRT and vi the config,
ssh root@192.168.1.1
vi /etc/config/network
to change only the following (indented) lines:
config dsl 'dsl'
option tone 'bv'
option firmware '/lib/firmware/vr9-B-dsl.bin'
config interface 'wan'
option device 'dsl0.7'
option username 'H1und1/ui1234-567@online.de'
option password 'xxxxxxxx'
Then I reboot, ssh again and install the GUI:
opkg update
opkg install luci luci-ssl
/etc/init.d/uhttpd restart
As the last step before configuring stuff like WiFi or login password I activate
Network -> Firewall -> Software flow offloading
because otherwise my connection becomes way slower (has nothing to do with your vectoring problem, though)
Ladies and Gentlemen;
DSL Status
Line State: Showtime with TC-Layer sync
Line Mode: G.993.2 (VDSL2, Profile 17a, with down- and upstream vectoring)
Line Uptime: 0h 0m 23s
Annex: B
Data Rate: 63.679 Mb/s / 12.736 Mb/s
Max. Attainable Data Rate (ATTNDR): 86.802 Mb/s / 38.838 Mb/s
Latency: 0.13 ms / 0.00 ms
Line Attenuation (LATN): 15.9 dB / 18.1 dB
Signal Attenuation (SATN): 16.4 dB / 18.0 dB
Noise Margin (SNR): 12.9 dB / 23.0 dB
Aggregate Transmit Power (ACTATP): 4.8 dB / 12.6 dB
Forward Error Correction Seconds (FECS): 0 / 3356
Errored seconds (ES): 0 / 1639
Severely Errored Seconds (SES): 0 / 563
Loss of Signal Seconds (LOSS): 0 / 0
Unavailable Seconds (UAS): 93 / 93
Header Error Code Errors (HEC): 0 / 0
Non Pre-emptive CRC errors (CRC_P): 0 / 0
Pre-emptive CRC errors (CRCP_P): 0 / 0
ATU-C System Vendor ID: Broadcom 193.214
Power Management Mode: L0 - Synchronized
For reasons I cannot comprehend it works now. I restarted twice and my Fritzbox 3370 seems to get reliable internet.
Klu had the right clue, I guess, but I don't know what is different now than before: I think my settings were the same, but I use:
OpenWrt SNAPSHOT r19709-4bed263af7 / LuCI Master git-22.137.71281-d6dbedd
now.
Could it be that this was the cause, I havn't used the snapshot image?
I will let the router do its way for next few days, test stability and then I guess this question is closed.
Thank you very much for your time, thoughts, and support. This is well appreciated and I will continue to do so in the fields I have expertise also.
Thanks!
neophrema
Glad to hear it's working now. Maybe you did a mistake somewhere without noticing and this time didn't?