Modem mode issues

I have been using modem mode on my Virgin hub for years, and using a Linksys router for DHCP, wi-fi etc.

However, due to incredibly slow speeds I have contacted VM for help and they say they that it is not possible to check everything with the hub in this mode.

I checked how to modify the setting back to router mode and tried their solution of logging into the hub using 192.168.100.1 but I am unable to do so because it reports as not found.

I tried setting my PC to 1925.168.100.2 as per others suggestions but still no joy.

further investigation found 192.168.0.1 as an alternative but once again, no joy.

Has anybody experience this problem and if so, how do you fix it?

I was initially reluctant to change the hub back to standard mode but I am increasingly frustrated by the low speed internet I am suffering (despite paying for much higher speed) but if it is the only solution, then I must try.

Any suggestions?

Thanks for reading so far.