Compare wired and wireless performance
If possible, test a computer connected to your router by Ethernet, then compare Wi-Fi nearby and in the rooms where you have problems. Use the same test and similar conditions. Avoid running a large download during the comparison.
Give the router room
Try an open, central location rather than a cupboard or the floor. Walls, distance, and other equipment can affect the signal. A different location may help more than a faster internet plan.
Check the device and the network
Compare more than one device. If only one struggles, review its software and wireless hardware. If every device slows down, check for heavy downloads, backups, or other activity sharing the connection.
Consider equipment and coverage
Ask your provider whether your modem and router support the service you pay for. For hard-to-reach rooms, evaluate a better access-point arrangement or a mesh system. Placement and the connection between units still matter.
Escalate with useful evidence
If the wired connection also performs poorly, note the times, devices, and test results before contacting your provider. Ask them to check the line, the service status, and the equipment. A plan upgrade may not address a fault.