Not on the modems, issue the commands on the Linux box. Offload is a negotiated attribute and if one end does not request offload, neither end will do it.
That is very incorrect. All of the options you listed are internal to the network adapter itself and allow various optimisations in how the OS sends and receives data from the network adapter. None of those options changes the behaviour of the data on the wire, nor are they negotiated between anything else on the network (there are some options that are, like flow control, duplex etc, but these are not those) If something was broken about the way the network adapter or driver implemented those offloads (quite a common thing in the past) then it would be equally broken no matter what modem was used, and would manifest itself in other ways. James