internet speed issues

I purchased this router a couple months ago and I have a couple of anoyying issues thus far. Issues are:

1: Every other day I pretty much have to restart the router as the internet speeds are at a crawl.

2: I notice that the device Manger doesnt show all the devices that are connected. even when they are drawing bandwidth they show offline.

I am running the latest firmware V2.3.2.40. I have hard reseted the router a few times. no change. When i restart the router the devices do show in network traffic and in the manager. but after I dont know how long they disapear again. But i can careless about that. The main issue is the internet speeds. i have 300/25 and when the issue happens its at under 10/.05 so very very slow. What do you think?