Internet slows to 5-6 mbs

Hi there.

I have a very strange issue with my Cox service.

Every morning I have to reboot my router because my service slows to 5-6 mbs.   This is for all wireless connections and all Ethernet connections.

I have gigablast and usually get 900+. I’m using a Linksys WRT3200ACM router.

I thought my router was dying so I replaced it with a WiFi 6 Netgear one.

The issue continued.

I returned the Netgear and replaced with an ASUS WiFi 6 router.

The issue continued.

So I returned it.

The reboot of the router ONLY always fixes the slow speeds.

Cox says it my router and not them.

Three different routers all have the same issue.

What could be wrong?

Suggestions?

Thanks in advance.

Don