Orbi RBS50 lag spikes

Well, the obvious solution is to 'wire the satellite permanently', which is almost always impractical.  WiFi extenders and mesh WiFi systems were created to address the problem of installing Ethernet cables in existing buildings.

Alternatives to physical Ethernet cables include:

• Multimedia over Coax Alliance (MoCA).
  https://en.wikipedia.org/wiki/Multimedia_over_Coax_Alliance 
  Many houses have coax cable connecting almost every room in the house, often running along the eaves, through the attic, etc.  If coax cable is available, MoCA works very well.  A pair of MoCA adapters creates what appears to be an Ethernet connection.
• Power Line. https://en.wikipedia.org/wiki/Power-line_communication uses the house electrical wiring in much the same way.  It is more 'touchy'.  Adapters on the same electrical circuit give the best performance and there can be issues with certain circuit breakers.  It definitely does not deal with circuits on separate panels.  A pair of the highest capacity Power Line adapters would run about $100US.  (I use Power Line to connect two Tivo units to the internet.)
• Wireless Bridge.  Typically used to extend a network to a remote building (barns are the most common topic), wireless bridges also create the appearance of an Ethernet connection.

Users cannot choose which WiFi channels to use for the router-satellite connection.

One other possibility is to try moving the router and satellite around a bit.  Who knows???

If it was possible to figure out what was interfering with the router-satellite WiFi connection, it might even be possible to shield those WiFi radio antennas with a judicious placement of metal to block the interference.  (It probably IS possible, but I certainly have no clue on how to detect that interference or figure out where it is coming from.)