This device has only one radio and therefore it can only be a client to on AP.
Moreover, this device has very limited resources (4/32) https://openwrt.org/toh/hwdata/tp-link/tp-link_tl-mr3220_v1 and my recommended for future use with OpenWrt. Even if it happens to handle the load, due to its limited storage you can't update the firmware, which leaves it subject to new vulnerabilities.
How about this idea. One of the hotspots is a USB dongle and the other is via a mobile phone. I think this can work. I've not used the USB port with OpenWrt before. Also is there a way to keep Master AP active when the Client connection is missing? I currently have OpenWrt Chaos Calmer 15.05.1 / LuCI 15.05 so I should upgrade.
It's not clear to me what you are trying to do. You already have an onboard radio. Do you want to add another one as USB dongle and a third one using a mobile phone tethering? Neither is an ideal solution for a permanent setup.
The other putting is that to run those you are likely need to install USB suppor
t and drivers, and you have small storage space.
Travelmate package does that. But then again you probably won't have space.
Yes, Chaos Calmer is obsolete and doesn't calm chaos anymore. You should upgrade, but your device doesn't support the most recent releases due to storage and memory limitations.
My advise to you is to get another medium range router and demote your current one to an access point, where it's safer in your LAN area, and then you can do with upgrading it to the available release for that model.