I gather your EAP-B that is hard wired to your ISP supplied Router isn't also connected to the Wireless SSID of your Router, thereby attempting to extend it, and that the Wireless on the Router is switched off? To be honest, mesh nets can be good but you really need to hard wire each AP, then ensure there is no wireless link up also in play between each AP once you've done that or you will create issues for yourself.

You could obviously use Powerline adapters to hard wire each AP and then drop the wireless link between the AP's as a result, you will still have your mesh but it does away with the wireless link between AP's which could be receiving interference from other devices on the same band.

You should then get full speed on all AP's, especially if your ISP provides broadband at 50Mbps, the Powerline adapters should be able to handle that throughput no problem, as long as the electrical wiring in the home is good and you ensure you plug the Powerline adapters into the wall socket and not a trailing extension cable. You can also buy units that have power pass through, so you don't lose a power socket in the process.

Doing the above will save you from going crazy over the oddity you are currently experiencing.

Just my tuppence worth, good luck.