Then hit Download the get all the drivers and firmware for the Surface, including Marvell Avaster Wireless-AC Network Controller driver.
My previous experiences with the wifi in the SP3 and SP4 were similar - they were less reliable and slower on wifi than just about any other wifi device I've owned since the days of 802.11b/g. Scroll down to find your Surface devices, like Surface Book 2, Pro 4, and Pro 3. My other devices connect to the same network at 300 Mbps or better. This with a Unifi access point just 20 feet away. Disconnected and reconnected, back to 100+, with working internet. Waiting a bit, it came up to 13 but still no internet. Just now, my internet connection dropped, and I figured out it was because the wifi connection had dropped to just 6 Mbps. But basically anything would be better than the Marvell Avastar, which is anything but marvelous.
I recommend Intel 82xx or 92xx like most midrange and higher-end laptops on the market use. But seriously, Microsoft: get another wifi chip. Using a SP6 for about a week now and really like it.