Из server.log:
[2019-05-18T17:23:52,148] <inform-4> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=CONNECTED, last_inform=13, ext/stun_ip=192.168.0.241, dev_ip=192.168.0.241, up=222
[2019-05-18T17:23:54,479] <webapi-153> INFO dev - [state] dev[fc:ec:da:31:a1:4f] CONNECTED->UPGRADING, state_expire=1558214814
[2019-05-18T17:23:54,481] <webapi-153> INFO event - [event] AP[fc:ec:da:31:a1:4f] запланирован для обновления администратором [Phil]
[2019-05-18T17:23:54,500] <inform-4> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=UPGRADING, last_inform=2, ext/stun_ip=192.168.0.241, dev_ip=192.168.0.241, up=225
[2019-05-18T17:23:54,503] <inform-4> INFO inform - <<< [upgrade] dev[fc:ec:da:31:a1:4f] {"_type":"upgrade","version":"4.0.21.9965","md5sum":"f99564746043097afc756376a276147f","url":"https://fw-download.ubnt.com/data/unifi-firmware/420b-U7PG2-v4.0.21_2b9965-75d297982fa5409bbd2288bc5cd44668.bin","server_time_in_utc":"1558214634502"}
[2019-05-18T17:23:57,357] <inform-4> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=UPGRADING, last_inform=3, ext/stun_ip=192.168.0.241, dev_ip=192.168.0.241, up=228
[2019-05-18T17:24:03,973] <inform-3> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=UPGRADING, last_inform=6, ext/stun_ip=192.168.0.241, dev_ip=192.168.0.241, up=234
[2019-05-18T17:26:01,041] <inform-3> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=UPGRADING, last_inform=118, ext_ip=192.168.0.241, dev_ip=192.168.0.241, up=45
[2019-05-18T17:26:01,044] <inform-3> INFO dev - [state] dev[fc:ec:da:31:a1:4f] UPGRADING->CONNECTED, state_expire=0
[2019-05-18T17:26:01,047] <inform-3> INFO event - [event] AP[fc:ec:da:31:a1:4f] подключен
[2019-05-18T17:26:01,051] <inform-3> INFO inform - <<< [setparam] dev[fc:ec:da:31:a1:4f]: [_type, blocked_sta, server_time_in_utc]
[2019-05-18T17:26:10,142] <inform-3> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=CONNECTED, last_inform=9, ext_ip=192.168.0.241, dev_ip=192.168.0.241, up=54
[2019-05-18T17:26:10,488] <inform-2> INFO inform - от [fc:ec:da:31:a1:4f](, U7PG2, 4.0.15.9872): состояние=CONNECTED, last_inform=0, ext_ip=192.168.0.241, dev_ip=192.168.0.241, up=54
Я также проверил mongod.log, но там не было ничего полезного. Посмотрел логи web-доступа на edge-файрволе, скачивание прошивки видно, ошибок нет.