While it's not explicitly stated in your link, I expect your IoT device to use the esp8266 chipset from Espressif - mwlwifi and the Marvell 88W8964 wireless (the one in your wrt3200acm) have known (serious) interoperability problems dealing with these chipsets (which are very common in IoT devices). These issues (as well as the numerous other ones with the out-of-tree mwlwifi and its proprietary firmware) are very unlikely get any attention (let alone be fixed), after Marvell has sold their wireless division to NXP, even less since NXP has EOLed their non 802.11ax chipset.
At this point, the only real fix would be replacing the router with a different one coming with decent/ supported wireless hardware and drivers, respectively to use a dedicated AP instead, don't expect the wireless situation for the wrt3200acm to improve.
The SDIO based third radio, 88W8887/ mwifiex, on your WRT3200ACM might actually be a little more cooperative than the 88W8964 based main wireless. However, this comes at a cost - mwifiex is:
a) known to mess up regdom settings for your device, severely limiting the intersection of valid (5 GHz-) channels left by mixing different (hardware-) regdomains.
b) is SDIO based, so comparatively slow
c) has no antenna attached, so range and throughput are even worse than SDIO already suggests
d) was originally meant (but in practice never actually used at all) by the OEM to do continuous background DFS scans in the 5 GHz band, not to be actually used as active radio - so while it apparently can technically use the 2.4 GHz band, it's not tuned for that.