Hi, maybe I remember wrong, but is possible to have with OpenWrt, both 2.4 and 5 ghz at the same time under the same SSID with the same password? In order to let the device selecth the one is better without configury both.
In my configuration seems that the 5ghz is not started, see here
What I'm doing wrong?
J
What channel did you configure?
At least 5.000 GHz would not be a valid channel. In case of DFS channels, you'll have to wait 60s before a channel can be declared as "free" of DFS events, before it can be used and before the AP interface starts.
For starting I would recommend to try a non-DFS channel first, leave the DFS channels for optimizing - and be aware that with larger channel bandwidths not every combination is accepted.
To answer the question, yes you can and often should set the same SSID and password on both APs on dual band. But realize that a lot of clients don't make the best choice between them. I have seen my Android phone disconnect from a very strong 2 GHz AP in favor of a very weak 5 GHz one.
Yes check the global configuration (the top part of the page where you set the channel) of both radios. If country is left blank, some features may not work. Set the two-letter code for your country. After setting a country, save settings and the list of channels should update to those that are both legal in your country and supported by the hardware and driver. Choose one directly not "auto".
Also like @slh said, if you are still having trouble getting wifi to start, try setting 20 MHz. Higher bandwidths only work on some channels.
In most countries, that use DFS, the DFS channels are those between 52 and 140 inclusive.
And as good privacy practice, you may redact your MAC address when sharing your config online.
I just set the channel on auto, maybe is not a good idea?
I can't find dfs settings, I don't have changed a lot on wifi settings
Not the best idea.
Could you post here the output of this command?
uci show wireless
You can cover the mac-addresses and the Wifi keys.
Please use "Preformatted text </>" for logs, scripts, configs and general console output.
root@OpenWrt:~# uci show wireless
wireless.radio0=wifi-device
wireless.radio0.type='mac80211'
wireless.radio0.hwmode='11a'
wireless.radio0.path='soc/1b500000.pci/pci0000:00/0000:00:00.0/0000:01:00.0'
wireless.radio0.legacy_rates='1'
wireless.radio0.htmode='VHT160'
wireless.radio0.channel='auto'
wireless.radio0.country='IT'
wireless.default_radio0=wifi-iface
wireless.default_radio0.device='radio0'
wireless.default_radio0.network='lan'
wireless.default_radio0.mode='ap'
wireless.default_radio0.ssid='OpenWrt'
wireless.default_radio0.encryption='psk2'
wireless.default_radio0.key='***'
wireless.default_radio0.wps_pushbutton='1'
wireless.radio1=wifi-device
wireless.radio1.type='mac80211'
wireless.radio1.hwmode='11g'
wireless.radio1.path='soc/1b700000.pci/pci0001:00/0001:00:00.0/0001:01:00.0'
wireless.radio1.channel='auto'
wireless.radio1.htmode='HT40'
wireless.radio1.legacy_rates='1'
wireless.radio1.country='IT'
wireless.default_radio1=wifi-iface
wireless.default_radio1.device='radio1'
wireless.default_radio1.network='lan'
wireless.default_radio1.mode='ap'
wireless.default_radio1.ssid='OpenWrt'
wireless.default_radio1.encryption='psk2'
wireless.default_radio1.key='***'
wireless.default_radio1.wps_pushbutton='1'
Why not the best choice? I thought, auto is good, is the router that make a decision, and I'm in a place with many wifi around
DFS is a new world for me 
I just see here that my R7800 have some frequeces for DFS. I don't know how to use this information and how can be usefull for me, I'm learning. Any advice?
Yep, I noticed this too but not in my home, this's the very 1st route for me that have 5Ghz. My home is pretty small at max I've 2 wall in 5mt from the router, so I can't see any speed problem at home.
Why not auto? I chose auto right to let the router assign the best one. I thought that the router can see all signal around and choose the best one for my location.
Automatic channel selection is not yet implemented in OpenWrt. The auto selection just chooses the first legal channel.
Ok, thanks. There's a way to choose a good channel for 5Ghz?
Download an application on your smartphone that scans for Wifi,
Do a scan and check which channels are less congested all around the area that your access point will cover.
So no auto and then a scan on both 2.4 and 5 GHz to check.
Just to know, there's any plan to support auto channel selection on Openwrt?
Even if it did, you'd need a passive antenna to scan all the time for other networks.
Right!
I'm trying some settings with channel, I found that for the 2.4 there's more "overlaps" but for 5ghz there's very few signals around here. The funny is that I tried 112 channel and 160 Width and seems not working. If I look at the netgear article seems there's no DFS channels for CE for 160 Width. But 80 width and 112 channel work.
Any idea?
In 2,4GHz you can really have only channels 1, 6, and 11 without interference. But in a densely residential areas it is highly unlikely to find a channel without interference from other Wifi. Therefore you'll need to compromise and use smaller width or live with it.
5GHz is not so popular yet and there are more channels available.
If you select the 160 bandwidth, you'll cover 80Mhz below and 80 above the selected frequency. This means that if the selected channel is near the border of the DFS channels you'll have to follow the DFS rules.
So select a channel at the beginning (36) or the end (165) and lower the bandwidth to 80. That should not interfere with DFS channels.