@CB587 

Advice for Deco mesh: if you haven't done it yet, set IoT Network with 2.4GHz band only, with very simple SSID such as abcd and very simple password such as 12345

If that allows mower connect to Internet, now start making SSID/password more complex. 

You can also play with security settings on IoT network, from "no security" if it allows it and all the way upp through the list.

If IoT network requires security, set Guest Network which can run with no security and passwordless, try it with very simple SSID just to see if device will connect. Obviously, this is for troubleshooting only, don't keep open Guest network for more time than you need to run tests. 

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If nothing works, but you want to keep both Deco mesh and Landroid mower, here is not elegant but working option: get the least expensive 2.4GHz only WiFi router you could find, connect it to one of Ethernet ports of either Main or Satellite Deco, configure mower connect to that WiFi router.

Example of such WiFi router: TP-Link N300 WiFi Router (TL-WR841N), its price should be under $20 USD.