I assume that you are on FTTP?
LOS= Loss of signal.
Most likely a fibre fault somewhere, or possibly a fibre modem fault.
Make sure that the fibre going into the modem is not been bent or damaged, and that the connector has not come loose.
All you can do is to report it as a fault.
there is some yellow stuff between the wire going to the modem and the connector
It would be best to arrange a visit from Openreach, via the FTTP fault number.
I am not sure whay you mean by "yellow stuff"
i think that person you were talking to was talking about the small yellow cable that connects to the right side when looking behind the router.
my router also doesn't work by having that red light on the "LOS" bar.
i went all over the internet for help and the most helpful and common way to fix it is to get their services help. but im here to get help first.