LAN speed great, wireless starts high and drops

All, please bear with me, although I work with computer systems I will admit almost complete ignorance when it comes to h/w.

I use XFINITY - the 60 MBPS plan, and performance was fine until recently. On a wired connection, it still is, but I’ve been experiencing issuues with wireless connectivity of late. A few things of one:
- I’ve run several different speed tests on the wireless connection and with one exception, they start out at 40-60 MBPS and then drop quickly. Sometimes the upload speed is fine sometimes it doesn’t work well but it does not have spike/drop. Interestingly, the “in app” wireless test does not show the spike/drop off either.

I am happy to reset the modem and in fact have browsed through several answers on here, but I’m afraid I need rather explicit directions as to how to update any settings in the modem, or in fact do anything to reset it (other unplugging the power and cable for 2+ minutes, which I’ve done several times to no avail)

Appreciate any help you see fit to direct my way!