No access to OpenWrt while all the lights are on

I am French, sorry for my english...

I come to the community, because I have a problem, it's 3 or 4 times that happens.

I have a TP-Link Archer C7 or AC1750 v.4 router and a Huawei 3372s dongle set in Stick mode.

I had the problem a few weeks ago and reinstall OpenWrt, so I went from the Snapshot version to the version 18.06.1, which normally is stable ...

Everything worked perfectly after that.

And now this problem is back: all the LEDs on the router indicate normal operation (LEDs on: Power, Wifi, Ethernet, Dongle Huawei fixed light in turquoise blue)

But no connection, both in Ethernet with my 2 computers, with my phone Wifi, no internet connection is possible, but I do not understand that there is no connection to the interface Openwrt, neither with FireFox, nor with Putty.

The 2nd and 3rd time it happened, I turned off the router with its On / Off button for more than 5 minutes and it came back, but not this time .....?

Did someone have the problem?

And what should I do, still reinstall everything ... I hope not.

Thank you in advance for the answers.

First I would look into if I have DHCP problem. Because if you dont have a valid IP address assigned from the router, you cannot access anything, either LuCI or internet. If you still have the problem, first try to assign a static IP to one of your LAN devices, for example the pc connected to Ethernet. If after the static IP you get access to router, then it's definitely the DHCP issue. Try to restart the dhcp and dnsmasq and see if the problem gets resolved. If not then I am not yet experienced enough to solve your problem.

I, myself, have issue with DHCP on my router, now and then but restarting router or the dhcp and dnsmasq helps.

This sounds very reasonable. I'd say definitely assign static ip to one computer and see if you can ping the router, if you can ping but not connect that's further info of use. Perhaps ssh server crashed or http server. Could be a memory leak causing programs to be killed by the kernel for example

Thanks for faster answers.

I had a problem with DHCP, I put a fixed IP address, but it did not change anything.
I made a diagnosis under Windows 7 and he found me a solution automatically without knowing that it is the manipulation to perform.
How to reset to DHCP, can we program a diagnostic in schedules tasks

Well you cannot perform a diagnostic from scheduled tasks for DHCP problems or maybe you can if you are an advanced user and know your way around. In 18.06.1 there are still bugs that need to be addressed so it's likely a bug where DHCP server dies somehow and does not recover after that. Just go to startup tab and restart odhcpd and dnsmasq and you should be okay after that. I would say increase the lease time to at least 24 hours so you dont have to do this very often just once a day maybe.

I myself use a 10 minute lease time for DHCP and it works very well. I had DHCP problems a couple of times on 18.06.1 but I just restarted the device at first but then I only restart the above packages.