Please Help. Worked great for 2 weeks. Now half my devices only see 2.4g. Connection is slower and worse than with our old router we used for 8 years. not a tech guru so if someone could please give m

Had router 2 weeks & worked great. Started slowing up & going out 2 days ago. Have about 12 devices using it. Now about 4 devices can only see 2.4g. A brand new QLED Samsung tv only sees 2.4. complete