This thread started as a query about the MT7621 Netgear R6850 router and then evolved into several branches.
Frankly speaking, given the pace of development of OpenWrt, it would be strongly recommended to get a Mini PC or a Pi with 2GB or 4GB RAM. Storage can be increased by using bigger Micro SD cards. This is because OpenWrt is fast becoming a large OS. And routers with hardware that can support the OS cost a LOT.
Be ready to invest more than $100 for a router with OpenWrt, or Build your own.
Moreover, manufacturers keep making minor revisions, making their devices incompatible with OpenWrt. This makes commercially available routers a risky investment for OpenWrt installation.
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Returning to the OP, the Netgear R6850 router is a decent router + Wi-Fi AC Access Point (AP).
It is a fairly old router but made by the American company Netgear.
It has a dual core 880mhz CPU with 128MB RAM and 128MB Storage Memory.
As an off-the-shelf router with Netgear's OS, it works reliably well. Netgear has ensured reliability and performance. However, one of the BIGGEST annoyances is Netgear essentially forces users to create a Netgear Account to access the local Admin panel of the router.
Coming to the OpenWrt OS for Netgear, there are Mixed reviews. OpenWrt, especially above v23.05, can struggle with 128MB RAM devices. But this is true for any and all MT7621 routers.
Secondly, for the asking price, the Netgear R6850 is a little expensive for such an old device. There are better routers in terms of performance, but OpenWrt support remains uncertain.
The reliability, performance, and Wi-Fi range of the R6850, however, is quite good, especially, if you plan to set it up and forget about the router. It won't bother you or need attention, especially when using sub 200mbps plans.
However, if using OpenWrt with apps and services on a router with a high-speed plan is a priority, then skip this router. Invest a little more and get a better product with at least 256MB/512MB RAM.