Having never implemented the Johnson monitoring FW can't really advise on modem/router GUI remote access.
However, as someone who worked away from home until retirement and remotely monitored my line stat's, son's and daughter's line stat's on FTTC VDSL-2 service can suggest running DSLStats on a Raspberry Pi using VNC free edition, worked very well, still does.
Remote access is to DSLStats GUI running Raspbian basic desktop program on the RPi's, never had any occasion to access the modem GUI as such.
First implemented when the excellent MDWS service was active to upload router stat's 24/7.
RPi's used were Zero-W models powered directly from the modem/router USB socket and Wi-Fi connected.
There are quite a few detailed posts in the "Router Monitoring Software" section explaining how this can be achieved.
Appreciate that this option would involve additional outlay, however, a RPi ZW + basic case if desired shouldn't break the bank, currently listed @ £9.30 + £5.00 for the case.
(8/16 Gb. SD card also required)
Don't know how the Johnson's stat's interface compares with DSLStats but don't think it was ever designed to be as comprehensive.
