The question is not about getting a random number, but about seeding the random number generator with some entropy to ensure "more randomness", which is traditionally a problem in an embedded headless device, which can boot from a pretty static situation (and boots without a real time).
There are lots of discussions in the old OpenWrt bug tracker, new bug tracker, old forum and this new forum about that. E.g.
and e.g. this from year 2011...
https://dev.archive.openwrt.org/ticket/9631
Like I said earlier, OpenWrt has currently its own method of storing a new random seed after a boot for the next boot. But the importance of that is reduced after kernel moved to the new blake2 based randomness. (kernel does not even report the entropy count, like earlier)
There are also solutions like haveged to provide additional entropy during the router operations.
Ps.
I used "chaoskey", a hardware device in WRT3200ACM mwlwifi my router for a while, as mwlwifi did not provide entropy like ath9k 