packages are in the repo…

opkg list kmod-rtl*
kmod-rtl8187 - 4.9.109+2017-11-01-9 - Realtek Drivers for RTL818x devices (RTL8187 USB)
kmod-rtlwifi-usb - 4.9.109+2017-11-01-9 - Realtek common driver part (USB support)

Word to the wise - that’s a pretty old chipset - 802.11 b/g - using this can adversely impact your WLAN and neighbors on the same channel

Maybe - depend on your Asus RT-AC87U and what it’s capable of - some HW revs can do DFS…

From a client perspective - iw is definitely your friend here… you can do iw list to see how the 5GHz driver is built and configured… if you see channels with something like this… client is ok, but AP is not…

		* 5260 MHz [52] (22.0 dBm) (no IR, radar detection)
		* 5280 MHz [56] (22.0 dBm) (no IR, radar detection)
		* 5300 MHz [60] (22.0 dBm) (no IR, radar detection)
		* 5320 MHz [64] (22.0 dBm) (no IR, radar detection)

Getting/Setting region… again - iw is your friend…

# iw reg get
global
country US: DFS-FCC
(2402 - 2472 @ 40), (N/A, 30), (N/A)
(5170 - 5250 @ 80), (N/A, 23), (N/A), AUTO-BW
(5250 - 5330 @ 80), (N/A, 23), (0 ms), DFS, AUTO-BW
(5490 - 5730 @ 160), (N/A, 23), (0 ms), DFS
(5735 - 5835 @ 80), (N/A, 30), (N/A)
(57240 - 63720 @ 2160), (N/A, 40), (N/A)

phy#0
country US: DFS-FCC
(2402 - 2472 @ 40), (N/A, 30), (N/A)
(5170 - 5250 @ 80), (N/A, 23), (N/A), AUTO-BW
(5250 - 5330 @ 80), (N/A, 23), (0 ms), DFS, AUTO-BW
(5490 - 5730 @ 160), (N/A, 23), (0 ms), DFS
(5735 - 5835 @ 80), (N/A, 30), (N/A)
(57240 - 63720 @ 2160), (N/A, 40), (N/A)

iw reg set might be available… do the right thing and use the region that the AP is physically located - DFS is there for a very good reason…