From the OpenWRT webpage on the MR3020
Note: Many of these routers are marketed as a "3G travel router" but none actually include a 3G modem - the marketing term rather means that the OEM firmware supports a certain range of 3G/4G modems to be externally connected to USB because it contains drivers for those USB modems! Ignore that, because with OpenWrt ANY router with USB supports 3G/4G hardware …
So you need an USB modem stick for 4G connections as second "radio".
That said, switching between different ISPs based on the availability of one will probably require a custom script (at least I am not aware of any out-of-the-box functionality in OpenWRT for that).
Doable, but requires programming/scripting knowledge.
The question is mostly about is it possible to work both as WiFi client for one network and AP for another with single radio
In principle, yes: All TP-Link devices I know support this (as do most modern Wifi adapters for PCs). However, you'll very likely have to use the same channel. Details depend on the hardware,
iw phy can give you details, look for valid interface combinations.