R6300v2 slow wireless connection w/new modem

Hello all,

I have, as listed above, an R6300v2 router.  We've had it about a year, and until upgrading our modem yesterday, all was quite well.  Now, after dealing with Time Warner/Spectrum, we have an excellent wired signal of 70Mbps or more, but wireless is a whopping 8Mbps...if we're lucky.  Before Saturday, when the replacement happened, we were easily getting mid-30Mbps wirelessly.  I can get the full 70+ using a wired connection through the router, but as half of our devices are wireless, we need this connection to be much better.

Thank you for your time.