Dear Onemore,
Miscarriage can be caused by various factors, even with egg donation. Incompatibility between donor and parents is one of them, but not the only or most important one.
The risk of a chromosomal alteration is low, but present. Since you cannot create new embryos for ethical reasons if you still have embryos from this donor, one option would be to perform a PGT-A to sort out all aneuploid embryos. If the sperm is highly altered, this may also be a good approach.
Then check other factors such as thrombosis, endometritis, receptivity, any uterine diseases, etc.
Finally, to rule out immunological problems, you can have some tests to see if your body needs help to accept the pregnancy and then tests on your partner and the donor to check your compatibility. This last test is called KIR-HLAc (KIR sensors for you and HLA-C for your partner and the donor). If your KIR is "difficult", the clinic should find a donor with the right HLAc.
We usually look for more in-depth explanations after two negative embryo transfers.
I hope this helps you.
Yours sincerely
Dr. Zermiani