tbh i dont know much about cabels and internett. but maybe there is something wrong with your modem/router? or a cable between your routers/modems.
or a cable from the wall into your routers/modems.
probably isent but why not check?
also i had a problem earlier where a cable that goes into our house was broken. so we would get shit speeds when it was bad wheater outside, like rain, snow or blowing alot.
they came to do a check and saw the cable, it was like broken. it used to be 2cm in diameter now its like 5cm pretty funny i think.
but yee, check your cabels from the wall into the modems and cable that connects from a modem to another modem if you got that.
EDIT: ask a friend if you could borrow their modem, or take yours to them and test it out