The AiMesh is simpler to use and more intelligent, and does make life simpler when it works
Alternatively, you could invest on a couple of Ethernet Powerline adaptors (they use your homes electrical mains circuit to carry data) and move the first RT-AC68U further away from your Smart Hub, thus keeping your existing set-up


AiMesh is great for Streaming and Gaming I am sure you can find another RT-AC68U on Ebay for under £100
As for Modems, If you are on a VDSL connection, then the DrayTek Vigor 130 is excellent and around £70 and is BT approved.
If you are on a FTTP / on Demand/ FTTH connection etc, then just another RT-AC68U is needed
In a completely sane world, madness is the only freedom (J.G.Ballard).