It is possible/likely that the firmware update (& the need for an updated HG612 Modem Stats program) caused a number of 'stuck' HG612_stats.exe processes.
This in turn could be causing too many login attempts & blocking telnet access.
If that's the case, terminating any 'stuck' processes may well clear things out, thus allowing telnet access.
I have attached an updated version of HG612_stats.exe that MIGHT deal with the new firmware, but I must warn that it is not fully tested yet as my own modem hasn't received the firmware update.
I am now on the same bandplan as you, so it seems the DSLAM udate has already happened.