You cant use backups files for one router for an entire another version of the same device: v2 ≠ v4 in matter of hardware. You need to reconfigure manually and use the unziped backups as reference. Big jumps in Openwrt releases or big changes in certain packages tend to result in issues too.
Beside that, depending on what you are running on the archer c7v2 or v4 itself (sqm or qos?), i wouldnt except more than stable 90-100mbit on wan. The cpu on them is quite slow.
I would rather focus rebuilding the entire network. Reset everything and use the latest stable. Use the archer c7v4 as a main router, swap from wds to mesh to get better performance over wlan (beside wiki, here is a reference in matter of mesh: Setting up authenticated mesh with wpad-mesh).
If the c7v4 router still crap randomly out on wan or lan, then you will have to check the rest like cabling and any provider device. If the v2 itself dies out as a mesh node, well its time for the bin. I had quite a bunch of them for an hotspot project and they all died out after a few years with similar issues (wan, lan and wlan getting unstable).
Do you have any info on the mikrotik? If its an active device, mikrotik tend to have breaking bugs in the past, and since isps arent trustable to update their own damn equipment...