How to setup 5GHz network on Archer C20 v1 with OpenWRT onboard?

Question

WiFi

There is an Archer C20 of the first revision. About 2 months ago I reflashed it to OpenWRT, + - set it up and started to enjoy it. Somehow I remembered about his 5GHz network - I tried to connect - I didn’t see the phone, like other devices, well, I scored. A month ago, I remembered about it, took a tambourine and began to shaman with the width, mode and channel, as it was advised on the Internet.
Phone with 5GHz support (Samsung A50)

According to the network settings at the moment, the situation is as follows:

Режим: AC
Канал: 36(5180 Mhz)
Ширина: 40 Mhz
Максимальная мощность передачи: умолчания драйвера (12 dBm)
Код страны: RU
Разрешено использование стандарта 802.11b.
Сеть в режиме точки доступа(как и вторая на 2.4 ГГц)

Shamanism did not lead to any result. Maybe I screwed up somewhere in particular? In which case, I have a backup of the current settings, so I can go all out.
Thanks in advance