Well, you have more than one thing mixed up.
You are seemingly following the method in the post lurker888 suggested, but when you follow a method, follow it completely.
The Teltonika devices that you connect (respectively) to ether2, ether3 and ether4 will have address 192.168.1.1.
So assign to ether2, ether3 and ether4 addresses 192.168.1.2/24, 192.168.1.3/24, 192.168.1.4/24 (you can add to all of them the same address, say 192.168.1.100, but it will be easier to get confused).
Then you are connecting to the Hex S via ether1, so you need to assign to ether1 192.168.88.1.
You have NO bridge in your configuration (which is good) but then you assign IP addresses 192.168.88.1, .2, .3 and .4 to the bridge (with a /32 netmask, that makes little sense).
You have the mangle rules in pre-routing that mark connections with destination 192.168.88.2, .3 and .4, OK.
Where are the other mangle rules?
You have the routing rules that send marked packets to the specific table, OK.
You have the dst-nat rules that change on the fly the destination addresses to 192.168.1.1, OK.
Compare again your setup with the one linked to by lurker888 or - possibly easier - with the “final” version posted here: