my modem is also zte 831
but i cant see any page with DSL parameter
i followed your steps.
WAN -> DSL -> DSL param.
no see DSL. section
Status -> ADSL line got la.

line status as seen here got these. also no see SNR margin .

ADSL Line Status

Current adsl line status is as the below.

Line Mode G.dmt Line State Show Time
Latency Type Fast Line Up Time Duration 05:21:14:40
Line Coding Trellis On Line Up Count 4

Statistics Downstream Upstream
Line Rate 512 Kbps 256 Kbps
Attainable Line Rate 1248 Kbps 896 Kbps
Noise Margin 13.9 dB 24.0 dB
Line Attenuation 63.0 dB 31.5 dB
Output Power 11.8 dBm 15.4 dBm
K 17 9
R 0 0
S 1 1
D 1 1
Super Frames 29909812 29909810
Super Frame Errors 2314 4
RS Words 0 0
RS Correctable Errors 0 0
RS Uncorrectable Errors 0 0
HEC Errors 718 0
OCD Errors 0 0
LCD Errors 0 0
ES Errors 0 0

thanks!