It might be the same upstream kernel related bug as discussed in R7800 exploration thread:
for ipq806x/R7800 it looks like 4.14.44 kernel worked ok but the bump to 4.14.48 broke things, and the 4.14.50 currently in 18.06 is still broken, but 4.14.51 will fix things. That .51 will likely hit the 18.06 rc2 or final.
You might test the .51 patch from mailing list:
https://patchwork.ozlabs.org/patch/932599/