User wnienhaus on Gitub posted a fix at https://github.com/libimobiledevice/libimobiledevice/issues/1038#issuecomment-735213839 that worked for me on openwrt 19.07.4 running on a router EA6350.
This modifies the iphone binary to change all references from 1516 buffer size to 1514 buffer size. Here are the instructions:
opkg update
opkg install kmod-usb-net-ipheth usbmuxd libimobiledevice usbutils
opkg install xxd
cd /lib/modules/$(uname -r)
cp -ia ipheth.ko ipheth.ko.orig
xxd ipheth.ko.orig | sed 's/ec15/ea15/g;s/ecc5/eac5/g;s/ec75/ea75/g' | xxd -r > ipheth.ko
rmmod ipheth; modprobe ipheth
opkg remove xxd
After this change, I was able to tether and ping on an iPhone XR running iOS 14. The user notes that the changes to the binary representation may not be the same with your router, so it depends on the underlying architecture.