Unfortunately this does not work, the TP-Link Archer MR600 can use the OneMesh Range Extener, but using a second MR600 as a mesh is technically not possible, you cannot set up a OneMesh system between two routers.

I would solve it like this: use an LTE router as a "modem" and switch off the WLAN there. Unplug all devices connected to the router and then buy a real mesh system. The TP-Link Deco Mesh Routers are very suitable for this, because each Deco device is included in the set, i.e. one of them is then connected to the LTE router as a master and the mesh network is then simply expanded with additional devices from the set.

I recommend the Deco M5 set as a mesh extension, it doesn't cost too much and also offers HomeSave free antivirus and antimalware protection that covers all devices that connect to the mesh router.

TP-Link Deco M5 Dualband WLAN Mesh 3er Set (AC1300)