Something has caught my attention. I'm wondering... assuming your line supports G.INP if, perhaps, the modem in question doesn't support G.INP and is falling back to a 'compatibility state'... that being traditional interleaving (INP/delay). I say this because I assume that your latency values are 'delay' values, typically seen on traditional interleaving, although odd values I must admit but that could be down to the modem. This is speculation though as I don't know what your other modem reports, and I could be wrong about the 'latency' values. Even so it's odd that it won't go higher than 62Mbit~ downstream given the spare noise margin. Assuming you go back to the other modem, am I right in understanding that it resyncs at full speed without a DLM reset?
The Sky router (ER110) will sync at the same speed but normally (overnight) it will then sync back at 79, just tried the Annex B vectoring firmware and the max attainable rate has dropped
Line State: Showtime with TC-Layer sync
Line Mode: G.993.2 (VDSL2, Profile 17a)
Line Uptime: 0h 0m 43s
Annex: B
Data Rate: 62.675 Mb/s / 20.000 Mb/s
Max. Attainable Data Rate (ATTNDR): 62.910 Mb/s / 25.317 Mb/s
Latency: 7.00 ms / 6.00 ms
Line Attenuation (LATN): 12.3 dB / 10.7 dB
Signal Attenuation (SATN): 12.3 dB / 10.6 dB
Noise Margin (SNR): 9.2 dB / 6.6 dB
Aggregate Transmit Power (ACTATP): 0.6 dB / 0.6 dB
Forward Error Correction Seconds (FECS): 184 / 77917
Errored seconds (ES): 0 / 2887
Severely Errored Seconds (SES): 0 / 111
Loss of Signal Seconds (LOSS): 0 / 2088
Unavailable Seconds (UAS): 127 / 127
Header Error Code Errors (HEC): 0 / 0
Non Pre-emptive CRC errors (CRC_P): 1 / 0
Pre-emptive CRC errors (CRCP_P): 0 / 0
ATU-C System Vendor ID: Infineon 178.6
Power Management Mode: L0 - Synchronized
I assume the right firmware is VDSL over ISDN Annex B firmware, which is confusing as I thought ISDN was dead
Here is the router stats from the Sky router, which I just plugged in and is syncing at the same speed, no idea why there's DS1, 2 and 3