It is time to get a new device. OpenWrt 15.05 is ancient and has many security vulnerabilities. The device itself (assuming is is < v3.0) has only 4MB flash + 32MB RAM. It can run LEDE 17.01, but that is only marginally better than 15.05 WRT being old, unsupported, and vulnerable. I'll get to another reason that new hardware is a good idea in a moment...
I had one of these that I used as my travel VPN router. At the time, I had created scripts to read the switch position and make changes to the configuration appropriately during boot (AP only, AP + client, and the 3rd position in my case was AP + client + VPN autostart). I can't remember exactly how I did this -- I recall that I had 'known good' files (network, wireless, firewall) saved on the device and the scripts simply copied the appropriate files into place on boot; I don't remember how I read the switch positions, but that might have been a hotplug.d function if it existed in 15.05.
The reason you are experiencing the issue is 2-fold:
when the radio is used for AP + client (sta) modes, it must establish the upstream (sta) connection in order to select the correct channel, which it will then use for the AP mode. If it cannot establish the link to the upstream AP, the radio simply can't start because it doesn't know what channel to use.
-- Travelmate deals with this by simply attempting to connect to a list of known wifi APs (in priority order)... each time one fails, it tries the next, and when the list is exhausted, it flips the device back to AP mode only.
this device has only a single radio, so there is literally no other option for wifi when the upstream network (via wifi) isn't available and the radio is in AP+sta modes.
-- This is the other reason new hardware would be a good idea. The TL-WR902AC is the successor to the MR3020 and has 2 radios --2.4GHz and 5GHz. This means that you can dedicate one to uplink (client/sta) and the other to AP mode. There are other travel routers on the market that also have multiple radios and even other features that the WR902AC doesn't have.
-- In addition, modern hardware means more memory, storage, and processing power which means you can run a modern/up-to-date version of OpenWrt (21.02 as of this writing). This, in turn, means that you are running a more secure platform, and have access to packages like Travelmate.
I'm asking for make or modify config for use AP + client mode. There was link, I use this for configure openwrt to work AP+client or only AP mode. But after reboot router, if there are hotsopt switched on from my phone, router isn't connecting to AP from my phone, client mode in openwrt settings is switched off.