There are some that have builtin LTE modems
I don’t know viable, available or pricey these are but I found the following in the table of hardware and there are still about as many of them left I didn’t copy.
https://openwrt.org/toh/views/toh_extended_all
COMFAST CF-E7
Edge-corE OAP-100
GL.iNet GL-AP1300
MikroTik RBwAPGR-5HacD2HnD&R11e-LTE (wAP ac LTE)
MikroTik RBwAPR-2nD (wAP R)
Sony AI Home Gateway (NCP-HG100)
ZBT WE1026-5G
ZBT WE1026-H
Arcadyan / Astoria Easybox 904 LTE
BOLT Arion
BOLT BL100
Cell C RTL30VW
https://openwrt.org/toh/views/toh_extended_all
As the total number user of alternative phone OS shrinks, the development of those OS becomes less and less viable. But yes, I would see phones with mid tier specs from 3 years ago charging flagship prices to fill that the spot before it becomes entirely non-viable. I think google does need there to be the appearance of competition so they don’t get hit with monopoly lawsuits, but it only needs to exist, other than that they want it as fringe and clunky as possible and I think that’s where we’re headed.