Com 5 is IRQ 4 --- Com 2 should be

COM2 is usually IRQ 3 & I/O Address 2F8h.
Knowledge Base should have IRQ & I/O assignments for your specific system.

Try moving the us robotics off of com1 to another com port /irq-com1 should be irq 4 and com2 should be irq 3 and allow the new equipment to use com1. I have found win modems and other devices to battle over com ports and not be a Dell issue. If the modem is a win modem then try a different type modem to free up the com port.