Seconding the above.
I will add the suggestions that you first establish one working Ethernet connection to the router. Ensure that that the connection is fully working and at the contracted or expected speeds. Just one device. Ensure that the connectivity is stable.
Then add a single wireless device and ensure that that device connects at the expected speeds and remains stable.
Add other devices as required and necessary but only one device at a time and ensure that all other devices continue to work and remain stable. With respect to both network and internet connectivity.
After that expand as necessary into repeater, mesh systems, or powerline networking.
Work from simple to complex. If you have a "path" that is proven to work at speed and is stable then you will know that some problem is probably a consequence of the current device being added. Likely either the device itself or some other issue created when the device is added.
Work from a plan. Sketch it out. Does not need to be fancy or a work of art. The plan will change....
All too easy to end up going in circles without a thoughtout and methodical installation plan.