iPad Pro 11 Wi-Fi issues when connecting an HDD

Connecting an external hard disk drive (HDD) to an iPad Pro 11-inch (4th generation) can cause Wi-Fi or Bluetooth interference, frequently happening because the USB 3.0/3.1 data stream or unshielded cables/adapters emit radio frequency interference (RFI) right around the 2.4 GHz Wi-Fi band.

Please try this

• Switch to 5 GHz or 6 GHz Wi-Fi: Go to your router settings and connect your iPad Pro to a 5 GHz (or Wi-Fi 6/6E 6 GHz) network instead of 2.4 GHz. Higher frequencies are generally immune to USB 3.0 interference.
• Use a Powered USB-C Hub: Connect your HDD through a powered USB-C hub (one that plugs into a wall outlet) rather than pulling all bus power directly from the iPad Pro.
• Use Shielded, High-Quality Cables: Replace generic or unshielded USB-C/USB-3 cables and adapters with well-shielded, branded accessories.
• Create Physical Distance: Use a longer USB-C extension cable to move the physical hard drive and its enclosure away from the top/side edges of the iPad Pro where the internal antennas are located.
• Switch to a Solid-State Drive (SSD): Mechanical HDDs vibrate and draw more continuous/spindle power than flash-based SSDs, making SSDs a more reliable option for tablet workflows.

I have a router only with 2.4 GHz mode, but I changed the channel settings from auto to 11 and now everything works without problems.

It temporarily worked on another channel, then it stopped, so I'll have to change the modem.