Connected without internet

Hello,

Please help me, no matter how hard i try seeking answers here, nothing works.
We have a pldt home wifi Model D2K-FT10. Its less than a yr old. One time, im connected to wifi and suddenly can no longer watch videos etc. I power off, reset and reset wps. But nothing works. Please help me, im having online class. The only way i can access to internet is get the sim out (smart rocket sim) the modem and insert it to my phone. Then use hotspot to connect it to my laptop. In which the connection is so slow since im just using mobile data. Icant afford buying new modem now. Please help me, thank you!