Self compiled version 18.06-r7930-bd3b8480ab on WDR3600 reboots when any wifi device tries to connects to router via 5GHz. Version ea8f7d74ba (date 12.11.19) works fine (self compiled too).
Logs of the failure would help understand the issue.
Do you get the same effect with tl-wdr3600-v1-squashfs-sysupgrade.bin from https://downloads.openwrt.org/releases/18.06.5/targets/ar71xx/generic/ ?
No. WiFi works fine. Same with self compiled from sources "openwrt-18.06" 12.11.2019. Actual build bd3b848 reboots.
I don't see anything "super obvious" between those commits in OpenWrt itself
$ git log --pretty=oneline ea8f7d74ba..bd3b8480ab
bd3b8480ab (openwrt/openwrt-18.06) kernel: bump 4.9 to 4.9.205
2777947a75 kernel: bump 4.14 to 4.14.156
7863a8f302 base-files: config_generate: split macaddr with multiple ifaces
daed78ab55 kernel: nf_conntrack_rtcache: fix WARNING on rmmod
8f6debf633 kernel: nf_conntrack_rtcache: fix WARNING on forward path
72ddeffc09 kernel: nf_conntrack_rtcache: fix cleanup on netns delete and rmmod
70b73f6470 kernel: Add missing configuration option
6e2e5d1bf8 kernel: bump 4.14 to 4.14.155
f6f916b3e5 ar71xx: fix buttons for TP-Link TL-WDR4900 v2
0e85ace840 ar71xx: fix LED setup for TL-WDR4900 v2
45fefa0459 ramips: set uImage name of WeVO 11AC NAS and W2914NS v2
b0d99e32db ar71xx: fix MAC address setup for TL-WDR4900 v2
22fe68643f ar71xx: fix MAC addresses for Archer C5 v1, C7 v1/v2, WDR4900 v2
2219b0258d ipq40xx: fix build error
0a9147be6a kernel: bump 4.14 to 4.14.154
b9e685eed5 kernel: bump 4.9 to 4.9.202
8179ac3dc1 ar71xx: fix WLAN LED names for Archer C7
6962c1e495 ar71xx: fix system LED names on Archer C5/C7
e6a7eacfea mac80211: brcmfmac: fix PCIe reset crash and WARNING
f65330d27d ramips: assign correct key-code to wps buttons
ab6addc95d ramips: rt3833: fix build breakage
f503bc3d25 ramips: add usb-ledtrig-usbport to DEVICE_PACKAGES of CY-SWR1100
506bfaa126 ramips: fix MAC address setup for Samsung CY-SWR1100
c89437e398 gitignore: ignore patches in OpenWrt root directory