I am having the exact same issue in Portsmouth.
I had my BT Business broadband upgraded from an old account to a new "superfast account" on the 17th March.
Since the 18th of March I've had drop outs every night between 12:15am and 3:15am.
I have a static IP address assigned by BT and I'm also using a DSL-AC88U
I've not reported the issue yet, as I thought it was a symptom of being a new upgrade.
I had to have a new box installed. (Side note, my speed was meant to increase and it's actually gone DOWN... but that is another story!)
Here is my router logs for tonights blip ( just now!! )
Apr 11 01:00:16 DSL-AC88U daemon.info dhcpd: Reclaiming not used lease 192.168.1.151.
Apr 11 01:00:16 DSL-AC88U daemon.info dhcpd: DHCPREQUEST for 192.168.1.151 from 50:50:a4:18:62:ca (Galaxy-Tab-S6) via br0
Apr 11 01:00:16 DSL-AC88U daemon.info dhcpd: DHCPACK on 192.168.1.151 to 50:50:a4:18:62:ca (Galaxy-Tab-S6) via br0
Apr 11 01:27:59 DSL-AC88U daemon.info dhcpd: Reclaiming not used lease 192.168.1.131.
Apr 11 01:27:59 DSL-AC88U daemon.info dhcpd: DHCPREQUEST for 192.168.1.131 from 18:69:d8:2b:43:80 (wlan0) via br0
Apr 11 01:27:59 DSL-AC88U daemon.info dhcpd: DHCPACK on 192.168.1.131 to 18:69:d8:2b:43:80 (wlan0) via br0
Apr 11 01:53:43 DSL-AC88U user.notice VPN: OpenVPN server stop
Apr 11 01:53:43 DSL-AC88U daemon.err openvpn[22737]: event_wait : Interrupted system call (code=4)
Apr 11 01:53:43 DSL-AC88U daemon.notice openvpn[22737]: Closing TUN/TAP interface
Apr 11 01:53:43 DSL-AC88U daemon.notice openvpn[22737]: /usr/sbin/ip addr del dev tun21 10.8.0.1/24
Apr 11 01:53:43 DSL-AC88U daemon.notice openvpn[22737]: SIGTERM[hard,] received, process exiting
--------- Lots more openVPN logs -------------
Apr 11 01:53:44 DSL-AC88U daemon.notice openvpn[362]: Initialization Sequence Completed
Apr 11 01:53:47 DSL-AC88U daemon.info pppd[20492]: Exit.
Apr 11 01:55:29 DSL-AC88U daemon.notice pppd[2382]: pppd 2.4.1 started by root, uid 0
Apr 11 01:57:57 DSL-AC88U daemon.info dnsmasq[318]: exiting on receipt of SIGTERM
Apr 11 01:57:59 DSL-AC88U daemon.info dnsmasq[5200]: started, version 2.78 cachesize 150
Apr 11 01:57:59 DSL-AC88U daemon.info dnsmasq[5200]: compile time options: IPv6 GNU-getopt no-DBus no-i18n no-IDN DHCP no-DHCPv6 no-Lua TFTP no-conntrack no-ipset no-auth no-DNSSEC no-ID loop-detect no-inotify
It appears that the router "reboots" however, during the offline time I am able to access the router... so I'm guessing it's a BT network reboot and not an actual router reboot???
