If you have a leased line or cable running directly to your SLIP server, then there is no need to use dip to initiate a connection. slattach can be used to configure the SLIP device instead.
In this case, your /etc/rc.inet1 file should look something like the following:
#!/bin/sh
IPADDR="128.253.154.32" # Replace with your IP address
REMADDR="128.253.154.2" # Replace with your SLIP server address
# Modify the following for the appropriate serial device for the SLIP
# connection:
slattach -p cslip -s 19200 /dev/ttyS0
/etc/ifconfig sl0 $IPADDR pointopoint $REMADDR up
/etc/route add default gw $REMADDR
slattach allocates the first unallocated SLIP device (sl0, sl1, etc.) to the serial line specified.
Note that the first parameter to slattach is the SLIP protocol to use. At present the only valid values are slip and cslip. slip is regular SLIP, as you would expect, and cslip is SLIP with datagram header compression. In most cases you should use cslip; however, if you seem to be having problems with this, try slip.
If you have more than one SLIP interface then you will have routing considerations to make. You will have to decide what routes to add, and those decisions can only be made on the basis of the actual layout of your network connections. A book on TCP/IP network configuration, as well as the man pages to route, will be of use.