Surf SOHO MK3 Interferes with LTE Cellular

I deployed a new Surf SOHO MK3 a few months back. Got it up and working with my hard wired LAN and WiFi with VLANs. It replaced an Asus RT-AC3200 router which worked well except it couldn’t accept VLANs from an upstream pfsense device. The Surf SOHO MK3 lives in my home office in the same place previously occupied by the AC3200.

Soon after it came online, I couldn’t make or receive cell phone calls in my office. The cell phone worked fine in the living room and the rest of the house and outside. Once on a call, walking into the office made conversation impossible: neither party on the call could hear the other.

Acting on an educated guess, the 5GHz WiFi in the SOHO was disabled leaving only the 2.4 GHz WiFi operable. The cell phone immediately starting working as expected.

Be advised, the AC3200 never interfered with cell phone operation.

The configuration of the SOHO has been left with the 5GHz off so the cell phone can be used.

In the SOHO WiFi configuration Operating Country is set to United States, Protocol for 2.4 GHz is 802.11ng and for 5 GHz is 802.11n/ac. Channel and Channel Width is Auto and Output Power is Max. (BTW, I had far more WiFi range with the AC3200 and no cell phone interference. I believe it’s output power was set to 400mW.) All other WiFi Settings are at default values I believe.

I would like the 5 GHz to work, and hopefully be optimized, without cell phone interference. Any help or suggestions are appreciated.