Set connection speed slower with O2 Box 6431 with lantiq xrx200 possible?

Hello,

I am using an O2 Box 6431 with openwrt 19.07.3 and the connection is not really stable when the sun changes from shining low to bright and vice versa. That sounds crazy, but I already have wago-clamped the wires directly to the O2 box to mitigate any issues with electrical contacts.
But: when using the built-in firmware (although with the original box, not the exact same hardware) I get a better (although not completely flawless) connection, which also runs at a slower speed (you can see this when disabling Stylesheets): The slower speed itself is not a problem since the line speed is cut to 50MBit anyways (by some other technique O2 seems using).

Data Rate:

Upstream Downstream
Actual Data Rate 31997 (Kbps.) 98406 (Kbps.)

Operation Data / Defect Indication:

Operation Data Upstream Downstream
Noise Margin 10 dB 15 dB
Attenuation 2.7 dB 9.1 dB

When running openwrt I get something like this, depending on the firmware:

ATU-C Vendor ID: Broadcom 193.218
Line Mode: G.993.5 (VDSL2 with down- and upstream vectoring)
Profile: 17a
Forward Error Correction Seconds (FECS): Near: 0 / Far: 0
Data Rate: Down: 109.981 Mb/s / Up: 31.996 Mb/s
Line Attenuation (LATN): Down: 9.0 dB / Up: 6.9 dB
Signal Attenuation (SATN): Down: 9.0 dB / Up: 6.8 dB
Noise Margin (SNR): Down: 11.1 dB / Up: 13.2 dB
Aggregate Transmit Power (ACTATP): Down: -4.1 dB / Up: 14.5 dB
Max. Attainable Data Rate (ATTNDR): Down: 109.553 Mb/s / Up: 38.188 Mb/s

Is there any way to get a slower and more reliable connection? I already tried to set SNR offset by 2...3dB, but that didn't seem to have an effect.

greetings

aha, it seems I was not trying all the values there are in the list when setting Downstream SNR offset:
now with +4dB the connection I get seems more like the one the O2 box gets. That is quite strange, since I tried multiple values (larger and smaller) over the last day, and it didnt make much difference. Maybe it helped that the line was offline for more than 30minutes.

ATU-C Vendor ID: Broadcom 193.218
Firmware Version: 5.7.8.12.0.7
Line Mode: G.993.5 (VDSL2 with down- and upstream vectoring)
Profile: 17a
Forward Error Correction Seconds (FECS): Near: 0 / Far: 15
Data Rate: Down: 99.984 Mb/s / Up: 31.996 Mb/s
Line Attenuation (LATN): Down: 9.8 dB / Up: 7.4 dB
Signal Attenuation (SATN): Down: 9.8 dB / Up: 7.3 dB
Noise Margin (SNR): Down: 15.2 dB / Up: 12.9 dB
Aggregate Transmit Power (ACTATP): Down: -3.8 dB / Up: 14.4 dB
Max. Attainable Data Rate (ATTNDR): Down: 118.331 Mb/s / Up: 37.866 Mb/s

it seems that now the downstream speed sometimes drops too a kind of degraded state.
DSL speed is still reported as

Data Rate: 89.986 Mb/s / 31.996 Mb/s

but actual download speed drops to about 1Mb/s some times. Upload is still great and so is ping.
Reconnecting the ppoe connection does not fix this, but reconnecting DSL does.
Can this be a side effect of the setting of this downstream offset? Are other reasons likely?